Carswell said he was leaving “amicably” and that there was no need to call a by-election in his Clacton constituency as he will not be rejoining the Tories or switching allegiance to another party.
His decision to sit as an independent follows a long-running feud with former leader Nigel Farage, who claimed the MP was “never Ukip” and had “sought to undermine us”.
Ukip leader Paul Nuttall, who in February failed to get elected as an MP in Stoke-on-Trent Central, said Carswell was due to face a grilling by party chiefs over whether he joined the party to restrict Farage’s involvement in the EU referendum. Nuttall said Ukip had not benefited “financially or organisationally” from Carswell’s presence in the House of Commons and so his departure would make “no difference” to his reform agenda.
“This is not a surprise,” the leader said. “I was elected on a pledge to forge unity in the party, and have had many discussions with key players to try and make that happen, but it had become increasingly clear to me that some things were simply beyond reach.
“Douglas was genuinely committed to Brexit, but was never a comfortable Ukipper.
“On Monday, he had been due to meet with the National Executive Committee to answer various questions relating to longstanding issues, including published allegations that he had joined us in order to try to minimise Nigel Farage’s involvement in the referendum.
“As we redefine our mission and take up the next phase of our campaign to rebuild a confident, independent nation, Douglas would have been increasingly out-of-kilter with our members’ aspirations.
“We now have an opportunity to put behind us the most damaging internal conflict which has dogged us over the past year, and look forward with optimism and unity of purpose to the very real challenges of policing Brexit.”
Carswell immediately faced calls to trigger a by-election from Ukip’s biggest financial backer, Arron Banks.
Banks wants to follow through on his threat to stand against Carswell, but it is unclear whether he would be able to do it under Ukip’s banner as he is no longer a member of the party.