I've seen many political defections before, but this is different – we're on the brink of something momentous

It is surprising that it has taken so long after Corbyn’s election as Labour leader in 2015 and his re-election a year later for Labour to split

Defections are the hard currency of politics. In between elections, MPs’ changes of party reveal the tectonic shifts beneath the slowly changing landscape. Ian Austin is the latest, leaving the Labour Party to sit as an independent, although he is not joining the Independent Group (yet).

When I started as a political journalist before the 1983 general election, the Social Democratic Party had just broken away from Labour. I’m afraid I thought they were traitors and I had a particularly low opinion of Shirley Williams.

I liked Michael Foot, the Labour leader, and thought the new party would take votes away from Labour, even though I still thought Labour would win – people forget how unpopular Margaret Thatcher was until the Falklands war and economic recovery in 1982.

So I understand how Jeremy Corbyn’s supporters feel now, even though I was clear at the time that the politics of Tony Benn and the so-called hard left was very different from everything Foot stood for.

However, there was then a long gap in the defections story, as chronicled by Matthew Smith, a data journalist at YouGov, who has compiled a chart of them all from 1832 onwards. We pick up the tale after the death of John Smith, Foot’s successor but one, in 1994. By then I was at the BBC. I had been working on a biography of Tony Blair, having decided he was going to be leader one day – now that day was suddenly upon me and the publisher wanted a book.

In between Smith’s death in May and the Labour leadership election in July, which it seemed likely that Blair would win, a series of four by-elections were held on the same day. They are mainly memorable today for providing us with one of the finest pub quiz questions: which two cabinet ministers unsuccessfully fought by-elections in neighbouring seats?

The answer is Theresa May and Philip Hammond, who fought and lost the Barking and Newham North East by-elections in east London on 9 June 1994. What seemed more significant at the time, however, was that Alec Kellaway, the Liberal Democrat candidate in Newham North East, defected to the Labour Party before the by-election and urged his supporters to vote for Stephen Timms, the Labour candidate.

Kellaway was a harbinger of the great change in politics that was coming.

The following year, Blair was Labour leader, I joined The Independent and Alan Howarth became the first MP ever to switch straight from the Conservative Party to Labour. The Tories howled for him to stand down and fight a by-election in his safe Tory seat of Stratford-upon-Avon, but he ignored them and instead fought the safe Labour seat of Newport East at the 1997 general election.

Those who could not believe that Labour was going to win a landslide – and I won’t embarrass senior colleagues at The Independent who doubted it – should have paid attention to the defection index. For an MP to change party is a hard, emotional and personal decision and so any switch is an indicator of huge stresses on the structure of politics.

The next time that was true was when two Conservative MPs defected to Ukip in 2014. Douglas Carswell and Mark Reckless did fight by-elections, because the Ukip tide was running high and the government was unpopular. And they won them – although Reckless then lost in the general election the next year. But if you needed advance warning of the 2016 Brexit vote, there it was.

That needs to be remembered now. In many ways, it is surprising that it has taken so long after Corbyn’s election as Labour leader in 2015 and his re-election a year later for the Labour Party to split. But the 2017 general election forced the party to stick together temporarily on the unspoken understanding that Labour stood no chance of winning.

Now the split has come. This week has been a doubly historic one in Westminster because the strain of Brexit finally forced the break not just in Labour but in the Tories as well. To be honest, I didn’t believe that as many as seven Labour MPs would quit until I saw them all filing into the room at County Hall on Monday – including Ann Coffey, who served as Tony Blair’s parliamentary private secretary in 1997-98.

So, it is not clear what will happen now. We know that 23 of 28 defectors to the SDP lost their seats at the 1983 election. But the SDP had a huge effect on the Labour Party in particular, helping to push it away from the politics of Tony Benn and Jeremy Corbyn.

We cannot be sure what shape the Independent Group will take, but the history of defections suggests that we are on the brink of something momentous.