The BBC reports he is doing so “amicably”.
Arron Banks appeared to be quite pleased with the news.
Others not so.
Ukip leader Paul Nuttall has said the party has not “benefited financially or organisationally from having Douglas (Carswell) in Westminster”.
The move is a major blow to the already-embattled party which not only loses its only MP but also hundreds of thousands of pounds in funding.
It has been an extraordinary achievement. UKIP, my party, which was founded in 1993 in order to get Britain out of the European Union, has now achieved what we were established to do.
On Wednesday, the Prime Minister is going to trigger Article 50, beginning the formal process of withdrawing our country from the EU. By April 2019, Britain will no longer be a member of the EU. After twenty-four years, we have done it. Brexit is in good hands.
UKIP might not have managed to win many seats in Parliament, but in a way we are the most successful political party in Britain ever. We have achieved what we were established to do – and in doing so we have changed the course of our country’s history for the better. Make no mistake; we would not be leaving the EU if it was not for UKIP – and for those remarkable people who founded, supported and sustained our party over that period.
Our party has prevailed thanks to the heroic efforts of UKIP party members and supporters. You ensured we got a referendum. With your street stalls and leafleting, you helped Vote Leave win the referendum. You should all be given medals for what you helped make happen – and face the future with optimism.
Like many of you, I switched to UKIP because I desperately wanted us to leave the EU. Now we can be certain that that is going to happen, I have decided that I will be leaving UKIP.
I will not be switching parties, nor crossing the floor to the Conservatives, so do not need to call a by election, as I did when switching from the Conservatives to UKIP. I will simply be the Member of Parliament for Clacton, sitting as an independent.
I will leave UKIP amicably, cheerfully and in the knowledge that we won.
At the hundreds of meetings and action days I have attended as a UKIP activist across the country since I joined in August 2014, I have met some truly remarkable people. You are heroes! Thank you and well done. I wish you all well.
When first elected to represent Clacton in 2005, I promised to do all I could to help ensure that Britain left the EU. To the consternation of my then party whips (some of who, I’m delighted to see, are now ministers helping make Brexit happen), I made my intentions on that front plain in my maiden speech. Job done.
I will be putting all of my effort into tackling some of the local problems affecting the NHS in our part of Essex, including GP shortages and the threat to our local Minor Injuries Unit. In that spirit, I called a Westminster Hall debate last week about the future of primary care in our part of Essex. Local comes first.
Cheer up! The days when small elites can try to arrange human social and economic affairs by grand design are coming to an end. Change is coming - Brexit is just the beginning.
Carswell’s future in the party had been in doubt for some time and members of Ukip’s NEC who wanted to see him kicked out of the party have been compiling a dossier of evidence against the Clacton MP, that was to be presented at a meeting on this Monday.
The central claim is that Carswell defected to Ukip in 2014 as part of a plot concocted with Tory MEP Daniel Hannan to “neutralise” then leader Nigel Farage ahead of the EU referendum.
Both Carswell and Hannan confirmed the plot last year in the book ‘The Brexit Club’, but it is only now that Ukip is considering taking action against their only MP.
More to follow...