Is the party finally over for Ukip after Douglas Carswell, its only MP, resigned from it? He will sit as an independent without fighting a by-election in his Clacton constituency, as he did when he defected from the Conservatives in 2014. Although Mr Carswell has “no plans” to rejoin the Tories, he does not rule out standing for the party at the next general election, when it seems his round trip may be completed.

It is easy for Ukip’s critics to enjoy the latest headlines about its “civil war” and suggestions it should now be renamed the UK Irrelevance Party. The internal strife long pre-dates its triumph in last year’s EU referendum. There is no doubt that the threat that Ukip could deprive the Tories of victory at the 2015 election persuaded David Cameron to promise his ill-advised referendum. In the event, Ukip won only Mr Carswell’s seat, and we will never know whether Mr Cameron overstated the threat or defused it. Whatever the truth, Ukip will earn a place in the history books, a rare achievement for such a small party.

Not surprisingly, a rebel force with one basic cause has struggled to find new one since last June. The party seems to have declared war on itself, with the bitter infighting and personality feuds reaching new heights. Nigel Farage’s latest departure as leader inevitably left a huge vacuum. Diane James came and went in 18 days. The party has also lost MEP Steven Woolfe, its major donor Arron Banks and now Mr Carswell. Its leader Paul Nuttall did poorly in the Stoke-on-Trent Central by-election, when he should have at least run Labour close.

Theresa May’s actions since becoming Prime Minister have also contributed to Ukip’s problems. She pursued a hard Brexit when Ukip’s prospects would have been enhanced by foot-dragging or arguing that Britain should remain in the single market, even if it meant keeping the free movement of people. Ms May allowed Ukip no space on the domestic agenda, implementing its signature policy of grammar schools. So one of Ukip’s legacies – and it is not one we welcome – has been to push the centre of gravity of British politics to the right. Ms May might prosper in these circumstances while Labour offers no credible opposition, but she would be wise to live up to her own rhetoric about governing from the centre.

Despite Ukip’s existential crisis, it is too soon for its opponents to dance on its grave, as Labour and the Liberal Democrats are doing. The party still averages between 10 and 13 per cent in the opinion polls. Sadly, whatever changes to immigration rules emerge from Brexit, there will probably always be room for an anti-immigration party in British politics. With Mr Carswell gone, the Farage faction will be freer to push this option. Ukip’s nasty streak was again on show when it tried to exploit last week’s terrorist attack in Westminster. Mr Nuttall described radical Islam as a “cancer in our society”, while Mr Farage blamed multiculturalism and a “fifth column” living in European countries.