Former UKIP leader Paul Nuttall has quit the party in protest at the decision to appoint Tommy Robinson as an adviser.
Mr Nuttall said he was leaving because the party was being taken in a direction "harmful to Brexit".
The association with the former English Defence League leader would "appal many moderate Brexit voters," he said.
Mr Nuttall's resignation follows that of fellow former leader Nigel Farage, who expressed similar sentiments.
UKIP's leader in Scotland has also quit the party, accusing it of being "infiltrated by people with an alternative agenda".
Mr Nuttall, a former history lecturer from Bootle, Merseyside, said he would continue to work as an MEP for North West England.
He said: "Putting Tommy Robinson front and centre, whilst Brexit is in the process of being betrayed is, in my view, a catastrophic error.
"The party leadership and my MEP colleagues have been aware of my views on this issue for some time."
Mr Nuttall took over from Nigel Farage as leader of UKIP in November 2016.
He was later heavily criticised after admitting claims that he had lost close friends in the Hillsborough disaster were false.
Standing down in June 2017, he said the party was more relevant than ever and would continue to be "the guard dogs of Brexit".
He also said the party had "lost its way in recent times" and attacked its "clumsy, blundering approach" to tackling female genital mutilation and its "hardline anti-Islam messages".
Mr Nuttall joined UKIP in 2004 after running for the Conservatives in a council election in Bootle two years earlier.
He became an MEP for North West England in 2009 and served as UKIP deputy leader from 2010.
By Gavin Stamp, political reporter, BBC News
UKIP's change of direction is also being played out in the European Parliament, where Gerard Batten has quit the political grouping chaired by former leader Nigel Farage.
The party's nine remaining MEPs, and most of those who have quit the party like Mr Farage, are members of the Europe of Freedom of Direct Democracy, one of the smaller pan-national groups in the Parliament.
The grouping, whose other major player is Italy's populist Five Star Movement, has shrunk considerably in size in recent years due to defections.
Mr Batten has not yet signalled whether he will align himself with another group in his remaining time in the European Parliament.
The UK will no longer have any MEPs after Brexit. But, as it stands, the UK's existing 73 representatives will be expected to work for three weeks after the scheduled Brexit day - 29 March 2019 - to complete the full parliamentary term ahead of May's elections.
Some former UKIP members have chosen not to join other groups although Janice Atkinson teamed up with Europe of Nations and Freedom, whose members include France's National Rally, the new name for the Front National.