Former Ukip leader Nigel Farage said on Tuesday he was leaving the party as he was uncomfortable with the direction it had taken in recent months.
Mr Farage, as Ukip leader, put pressure on former prime minister David Cameron for a referendum on the UK's EU membership, and then helped lead the successful campaign to leave the bloc.
But after stepping down as Ukip leader following the 2016 referendum, Mr Farage has been critical of the party, which he cast as disorganised and poorly led.
"I am leaving Ukip today," Mr Farage wrote in an article on Tuesday for the Daily Telegraph. "There is a huge space for a Brexit party in British politics, but it won't be filled by Ukip."
Mr Farage criticised in particular a decision by the party's current leader, Gerard Batten, to appoint far-right activist Tommy Robinson as an adviser.
Mr Batten later tweeted: “I hear that Nigel Farage has resigned from Ukip.
“Nigel & I were founder members of Ukip in 1993. I have always given him full credit for his work in Ukip bringing about and winning the Referendum. However, I feel that he left Ukip in spirit after the Referendum.” – Reuters/PA