Alan Watkins: Who will tell Mr Brown that it's time to go?

Any support for the bombing of Iran would prompt a deputation from the men in suits, of the type who called on Mrs Thatcher

The latest hurdle for Mr Gordon Brown to surmount is, as we know by now, the Glasgow East by-election. My own view is that, if Mr Brown falls, his departure will have been brought about less by unrest in Scotland than by an assault on Iran.

Of course, it may never happen. It may not happen that the citizens of the constituency reject their old allegiance or that the United States administration decides it would be a good idea to drop a few precautionary bombs on Tehran or other strategic areas.

But Mr Brown had better prepare himself for either eventuality or, conceivably, both. Of both these possible outcomes, it is the attack on Iran which is, I think, the more serious for the Prime Minister in his relations with his colleagues and his party.

When Mr Brown went to see Mr George Bush in the United States a year ago, though he had no need to trot along in the first place, he nevertheless contrived to give the impression that he would keep his distance from Mr Bush. The then newly appointed ministers Mr David Miliband and Lord Malloch-Brown reinforced that impression with speeches which were, by Foreign Office standards, models of plain speech.

They were promptly called to order, within hours, by officials both of the Foreign Office and of Downing Street. Mr Brown himself has obediently followed the official line. Indeed, it follows his own Atlanticist preconceptions. In this as in so many other respects, he is a continuation of Mr Tony Blair not so much by other means as by exactly the same means. The diplomatic reports, for what they are worth, tell us that, while the French are firm for decisive action over Iran, and the Germans are more doubtful, the British remain the staunchest allies of the United States.

So far, this unease has not been reflected either in the Cabinet or in the parliamentary party. The talk is of the by-election; of somehow muddling through to the recess; of house prices, the price of fuel and the price of food.

But it was Mr Blair's support for – or his refusal to condemn – the war in Lebanon which was the single identifiable cause of his decision to resign, confirmed at the party conference of 2006. In the previous year we had a general election, and a group (not very impressive, it must be confessed) were admitted to the House of Commons who were uncontaminated by their marches through the division lobbies in support of Mr Blair's war. Besides, several longer-serving Members, who were there at the time, have expressed varying degrees of contrition. Ms Harriet Harman, when she was a candidate for the deputy leadership of the Labour Party (and whose success was one of the few correct predictions I have ever made), donned a penitential trouser suit and confessed she had been wrong, very wrong, to support the Iraq war.

I enjoy and look forward to Ms Harman's baroque appearances at the dispatch box when she is deputising for Mr Brown at Prime Minister's Questions. She is not doing this because she is deputy leader of her party (for Mr Brown has not appointed a deputy prime minister), but because she is Leader of the House. In Margaret Thatcher's time the late John Biffen, who was likewise Leader of the House, took PMQs when Mrs Thatcher was otherwise engaged, even though she had a deputy in William Whitelaw. However, under Mr Blair, it was Mr John Prescott, nominally the Deputy Prime Minister as well as deputy leader, who stood in at PMQs. He was never leader of the house.

The truth is that these politicians make up the rules as they go along. As far as I can see, Mr William Hague is shadow Foreign Secretary and nothing else. But it is Mr Hague who is put up against Ms Harman. She did not have quite such a successful outing against the former Tory leader as she had in their previous encounter. In particular, Mr Hague's reference to Mr Brown's being past his sell-by date, a reference to the Prime Minister's charge of over-catering on the domestic front, drew loud, even if artificially exaggerated, laughter from the Tory benches but silence from Labour. While Ms Harman's claim, that the airports of the land would be unable to cope with the exodus of males if she ever became Prime Minister, was just plain odd.

Now, let us have a look at the party's book of rules. In Mr Blair's time, rules fell out of fashion, even though it may be because he was himself a lawyer by training. An example was provided by Mr Charles Clarke's appointment as "party chairman" after the 2001 election, when the party had a perfectly good chairman of its own already.

Still, the rules say: "When the party is in government and the party leader is prime minister and the party leader, for whatever reason, becomes permanently unavailable, the Cabinet shall, in consultation with the NEC, appoint one of its members to serve as party leader until a ballot under these rules can be carried out."

The rules are different when the party is in opposition. If the leader becomes "permanently unavailable", the deputy leader succeeds temporarily. This was what happened when John Smith died in 1994, and Mrs Margaret Beckett became leader (not, she likes to point out, acting leader), until the election of Mr Blair could be completed. She was herself a candidate, as was Mr Prescott too. In 1976, by contrast, Harold Wilson continued as Prime Minister until James Callaghan went to the Palace.

Mrs Thatcher did the same, the old girl staying gamely at her post in No 10 even though she had relinquished her leadership of the party. Who will ever forget her wiping a tear from her cheek as she left Downing Street for the last time with Denis beside her in the back seat, looking slightly more cheerful? We are unlikely to witness such affecting scenes when and if Mr Brown departs No 10.

Nor has the Labour Party traditionally gone in for what have become known as the men in grey suits. The original phrase, for which I claim paternity, was "the men in suits". It was meant to indicate the Platonic essence of a suit, a Tory suit or a White's Club suit, often aggressively striped and usually in dark blue. Alas, the phrase failed to catch on, and the word "grey" was added unnecessarily.

The best the Labour Cabinet could do in the circumstances would be a deputation consisting of Mr Alistair Darling, Mr Geoff Hoon and Mr Jack Straw. The last of these is an old survivor, like the former Soviet foreign minister, Andre Gromyko, always there in the group photograph. Mr Hoon is the Chief Whip. Mr Darling might strike some as being like a condemned man offering the consolations of religion to the prison priest. But he and Mr Straw are the only survivors of Mr Blair's Cabinet of 1997, and Mr Darling has already had to put up with many indignities at the hands of Mr Brown.

My guess is still that it is not going to happen. But any support for the bombing of Iran will make Mr Brown's position even more perilous still.