The Ukip leader, Henry Bolton, should quit his post, senior figures from the party have said, after Bolton’s girlfriend was suspended from the party for a reported barrage of racist remarks including ones about Prince Harry’s fiancee, Meghan Markle.

Bolton, 54, who has been leader of the party for less than four months, came under increasing pressure to resign on Sunday after it was revealed that his girlfriend, Jo Marney, 25, had sent a series of messages to a friend in which she made the highly offensive comments.

The party’s chair, Paul Oakden, said Bolton now found himself with a difficult decision to make and he was expected to decide on Sunday what to do to “help remedy the situation”.

“I think it is very clear that Henry is increasingly in a position where he’s got some difficult decisions to make,” Oakden told BBC’s Sunday Politics. He said the party needed to be “behind our leader 100% in taking that battle forward”.

The party’s national executive committee will meet next Sunday to decide on the next steps, unless Bolton should decide to quit before that meeting. “Whether or not the party decides it is willing to give that support to Henry is for the party to decide,” Oakden said.

The Ukip MEP Bill Etheridge, an influential figure in the party’s libertarian faction, said Bolton should leave now. “I am calling for Henry Bolton to resign as Ukip leader,” he tweeted. “A quick resignation is now essential to allow the party to regroup and campaign properly for Brexit.”

Earlier on Sunday, Ben Walker, a candidate in the last Ukip leadership contest, also called for Bolton to resign, accusing him of having “deeply flawed judgment”.

Bolton’s relationship with Marney, for whom he left his wife, is already under investigation by party officials. Marney said she apologised “unreservedly” for the “shocking language” used in the messages, reported by the Mail on Sunday, but claimed they had been taken out of context.

Peter Whittle, the leader of Ukip’s delegation in the London assembly, called for Marney to be “expelled altogether” for the “disgraceful remarks”.

Oakden said he decided to suspend Marney’s party membership immediately after he was made aware of the messages. “Ukip does not, has not and never will condone racism,” he told the Mail on Sunday.

Bolton left his wife Tatiana, 42, before his relationship with Marney became public in early January.

The Ukip leader confirmed he had had a “change in my relationship status” in recent weeks and said he had already made clear on social media that he had recently been spending time “with somebody who has become increasingly important to me”.

Should Bolton resign, the party would need to seek its fourth leader since mid-2016, when Nigel Farage resigned after the EU referendum.

Bolton, a relative unknown, replaced Paul Nuttall, who left the role after the party lost swathes of council seats and failed to make any headway in the 2017 election. His predecessor Diana James quit after just 18 days in the role.

In a letter to members, Oakden said the party’s national executive committee ruling body had agreed to discuss the leader’s private life at a special meeting in January.

On Sunday, a teenage activist called on Bolton to back calls for Marney to be expelled from Ukip. He tweeted: “If Henry Bolton truly cares about Ukip he will publicly call for Jo Marney’s removal from the party – her words were blatantly racist and there should be no room for that in Ukip.

“Also it would be hypocritical to allow her to stay … because he said in the leadership election he was the candidate against racism and Nazis so she has to go or he and Ukip are doomed if we let this behaviour happen in the party.”

Bolton tweeted in reply: “Jo was suspended immediately upon us receiving this information.”

Marney said in a statement: “The opinions I expressed were deliberately exaggerated in order to make a point and have, to an extent, been taken out of context. Yet I fully recognise the offence they have caused.”