UKIP East of England MEP Patrick O'Flynn has quit the party in protest at leader Gerard Batten's decision to employ Tommy Robinson as an adviser.
Mr O'Flynn said that under Mr Batten the party had become "an impediment to the Brexit campaigning that I have energetically pursued for many years".
He attacked the decision to employ Mr Robinson, suggesting the UKIP leader had "an apparent and growing fixation" with the co-founder of the English Defence League.
Mr O'Flynn, who has represented the East of England since 2014, said that he was quitting to join the Social Democratic Party.
His departure follows criticism by ex-leader Nigel Farage of the decision to hire Mr Robinson, real name Stephen Yaxley-Lennon, in a role advising Mr Batten on rape gangs and prison reform. Mr Farage denounced Mr Robinson as a "thug".