Another senior Ukip member has left the party over the leader Gerard Batten’s decision to appoint the anti-Islam activist Tommy Robinson as an adviser and take the party in a hard-right direction.

Ukip’s former economics spokesman Patrick O’Flynn is the third MEP to leave in two months over Batten’s anti-Islam views and policies. Flynn said he would join the remnants of the Social Democratic party (SDP), which is now pro-Brexit.

Batten, who became Ukip leader in February after the disastrous tenure of Henry Bolton, has described Islam as “a death cult” and proposed policies including screening of immigrants from Islamic countries, and Muslim-only prisons.

Such views prompted two other MEPs, William Dartmouth and Bill Etheridge, to quit in protest. In a statement on his website, O’Flynn said the final straw for him had been Batten’s “apparent and growing fixation with Tommy Robinson”.

Robinson, whose real name is Stephen Yaxley-Lennon, is the founder of the far-right English Defence League (EDL), an anti-Muslim street movement. He has recently restyled himself as a campaigner against sexual grooming gangs.

After Ukip’s national executive delayed moves to allow Robinson to join the party – as a former member of the EDL he is barred under current rules – Batten appointed him last week as an adviser on grooming gangs and prisons.

O’Flynn said the delay had been intended so the party could instead focus on “making a broad offer to Brexit voters disillusioned by the betrayal of Brexit by establishment parties”.

He said: “Instead, at this vital stage of the battle for Brexit, he has done the opposite, appointing Robinson as his policy adviser and announcing a plan for Ukip to be centrally involved in a mass demonstration being planned by Robinson on the issue of Brexit. This is despite the last two street demonstrations attended by Gerard ending up with some demonstrators engaging in very ugly scenes.”

He added: “Without any mandate from the membership or the party’s elected ruling body to go down this path, Gerard is transforming what Ukip stands for and offers to voters. Many longstanding party members have already left as a result.”

O’Flynn said he would instead join the SDP, the successor party to the rump of activists who remained when the original SDP, founded by breakaway Labour centrists in 1981, merged with the Liberals in 1988 to form the Liberal Democrats.

While the original SDP was pro-European, the current incarnation, which won 469 votes in the 2017 general election, supports Brexit.

O’Flynn’s departure piles more pressure on Batten after Nigel Farage, Ukip’s most successful leader, said he would seek to oust him over the appointment of Robinson.