No longer joined at the hip: Why 'Tony's twin' is on his own

Jonathan Powell was by Blair's side for the whole 10 years in Downing Street, intimately involved in everything. Now the formerly reticent aide solves the greatest mystery of the Northern Ireland peace process – and reveals the true nature of his relationship with the boss

Somebody was missing when Tony Blair made his grand return to Parliament a few days ago. His other half. Not Cherie, but the tall, slightly stooping aide with the curly hair who was a constant presence at the Prime Minister's side during his decade in power.

Jonathan Powell, Blair's chief of staff, was once described as being "like his Siamese twin". But there was no twin present when Tony went back on Thursday, looking tanned and far more relaxed than on his resignation a year ago this month. So, where was Powell? "I was not involved," he says. "Played no part."

That's a real surprise. Powell was once named as the third most influential figure in the Blair government, after the Prime Minister and Chancellor. He was intimately involved in everything, from victory in 1997, through war, to the final days. But not now.

"I've been doing my thing for a year," says the 52-year-old. We're in the garden of his home in west London. His plaid shirt and jeans say he is relaxed, but his body language says edgy. "Tony's been doing his thing. He's pretty busy around the world, so he hasn't been bothering me that much."

Yes, but as recently as March, when Powell gave his first ever interview, he said he was still being peppered with emails from the old boss. He also said he had been "trying to unravel" the over-committed Blair diary. But that was not Powell's job any more. The reaction of those actually employed to keep the diary went unrecorded – but it was easy to imagine them telling the boss they agreed with Powell's confession that he sometimes says "extremely plonkerish" things.

So, is he still receiving three messages from Tony before breakfast? "Less so now." How much contact do they have? "Er," he says. "I find it hard to judge. I sort of see him once a month, and hear from him once a week. Most weeks."

That's nowhere near as much as I expected. "No. Much less now." Sounds like Tony has found himself a new Jonathan? He smiles, and looks down at his shoes. Loafers. The laces are coming undone. "Er. He's got a very competent staff. Yes. Absolutely. Better than Jonathan ..."

It would be easy to say he sounds rueful. It must be difficult to adjust to being out of the most prestigious loop of all. But there is not much emotion in his voice. Charm, yes. Cleverness, in floods. But emotion is not useful to diplomats like Powell, who was working for the Foreign Office when Blair recruited him in 1995.

Detachment is a family trait. His big brother Charles advised Margaret Thatcher. They were close to the two most loved and reviled PMs of all, but have never rowed about politics, apparently. The same is certainly not true of his wife Sarah Helm, journalist and author, who once worked for this newspaper in the Middle East. She felt so strongly about what her partner and his boss were doing in March 2003 that she went on the anti-war march. The elder of their two daughters went, too.

The shock and awe of such a fundamental disagreement would have blown away some relationships. "It was quite an interesting situation domestically, yes," he says with a grin. "Have you never had a robust discussion with your partner?" Not usually about my taking the nation to war. "Sarah has very strong views." No collateral damage then? "We're still together. You'd have to ask her." She is with the girls, inside their surprisingly small, early Victorian terrace house. Slightly intimidated, I let her be.

The couple had their first child in 1997, heightening further the euphoria of that landslide year. The second, two years later, was the subject of the only real piece of gossip anyone has managed to print about Powell. The story – told by the Mail to illustrate New Labour being drunk on power – suggested they had left their eight-week-old daughter with the "hat-check girl" at the Groucho Club in London. "Oh God," he says. "I don't want to go back into all that. Please? Talk to Sarah about it. You'll get a thick ear."

This time I dare. She strikes out only in words. They were not Groucho regulars but guests at a function. The woman was not a "hat-check girl" but someone helping to run the event. She offered to sit by the child while Sarah ate in the same room. The helper was a child play specialist in her day job. "Like all these stories," says Powell, "it is nonsense ... but there is a grain of truth at the heart of it."

The remarkable thing is that apart from that snippet, nobody ever knew much about the private life of one of the most powerful men in Britain, did they? "No. So should it remain." Not likely. Jonathan, who has two sons by a first marriage, did not marry Sarah until he was free of Downing Street last year. He also took a holiday and wrote a revealing book. Great Hatred, Little Room: Making Peace in Northern Ireland (Bodley Head) uses government papers and his own diaries to describe the process from the inside. It reveals a lot about his own personal journey, too. At first he refused to shake the hand of Martin McGuinness, former IRA commander turned Sinn Fein No 2. Powell's own father had been shot at by the IRA while with the RAF in Ulster (although, typically of the clan, he plays it down). His brother Charles had been on a death list. Then, when negotiations progressed, McGuinness was invited to the Cabinet Room in No 10 for the first time, and said: "So this is where the damage was done." He was talking about the Anglo-Irish treaty of 1921. But Powell was thinking of the IRA attack of 1991, when Charles dived for cover under the table. "Yes," he said, "that garden there is where the mortars fell." Emotion was put aside so thoroughly that McGuinness became a friend, and attended Powell's wedding.

He will talk about the book at the Althorp Literary Festival on Sunday (details at www.althorp.com). Such events allow him to be challenged in public for the first time. "A woman at Hay said she was ashamed to be a Powell because of what I'd done over Iraq."

He is not appalled or ashamed. He just wishes the discussion was less heated. I attempt a challenge, but his responses are practised and familiar. Mistakes were made. Lessons can be learned. There should have been a broader coalition, and more preparation for a long-term presence, he says. But it was right to intervene. If his wife can't get through to him, I won't.

The book says the chief lesson of the Irish negotiations is to keep talking to your enemy, even al-Qa'ida. It also reveals an answer to the great mystery of Ulster: why did Ian Paisley turn from the adamant Dr No to the amiable First Minister, cracking jokes with his former enemies? "He was almost explicit about it on one occasion," says Powell, who believes the epiphany came while Paisley was in hospital in 2004 for a serious illness. There was an operation and he nearly died. "He said to us that he nearly met his maker and had come out of it thinking, 'I don't want to go down as the person who just said no'. I think he had a near-death experience, and that changed his attitude in a big way. It was the most remarkable transformation I've ever seen."

Presumably, Paisley would credit the Lord? "It probably was God," says Powell, chuckling. "He and Tony had these amazing discussions. It was thrilling to sit outside the room and hear lots of laughter. They would be talking about grace and forgiveness, and all these things that are foreign to me. Then I'd go in and find all these religious tracts on the desk. There was even one for Leo."

Was Blair trying to charm Paisley by putting himself forward as a fellow man of faith? "Well, I think he was establishing a relationship of trust, at a personal level. The way into that was their common interest in religion. That was something they could talk about." And pray about together? "No. I don't think I have ever seen Tony pray."

For all his friendship with Blair, five years his senior, Powell was always the one sitting just outside the room. He has described Tony and Gordon Brown rowing like a couple in the stormiest days of a divorce, but said even their closest advisers were simply the staff. "Did I say that? It's quite a good observation." Not that he was a yes man, he insists. "Tony can take criticism. As long as you go along with his decision." You do as you're told? "You make your advice then keep your counsel, whatever happens."

Powell was raised for such a role. His father was an air vice-marshal. The youngest of four boys, Powell was shipped off to Singapore with his parents. "The other three came out about once a year. It was like being an only child." Are they closer now? "Not really. Roderick lives in America, I don't see much of him. Chris lives round the corner, but I don't see much of him! Charles is constantly on an aeroplane."

When Blair left No 10 last June, Powell took a job with the investment bank Morgan Stanley (while the boss now works for JP Morgan). He had to pass FSA exams. "That was hard. Like a foreign language." What does he do all day? "I don't want to go into it too deeply. They're not keen on my giving interviews. Basically what every banker does: look for business, do deals."

Presumably it is very well paid, even for a man who earned more than some cabinet ministers? "Erm. Yes." So what is the bank buying, if not his expertise? Contacts? "Oh yes. Of course. The people they employ all round the world are for that." Not, though, contacts in the British government. Is that because all his friends have left? No, he says, unwilling to attack (although our photographer later tells me Powell joked: "Have you noticed it has rained every day since Gordon Brown became Prime Minister?") He can't lobby ministers because the rules don't allow it.

This will be his last interview, he says. "I'm a banker now." Powell wants to be anonymous. His own man. He left politics when Tony did because he had to. "I had no democratic legitimacy. I had no independent existence." He looked like a conjoined twin, but could never be an equal. "My existence was only as a creature of Tony."

Not any more.