NI21 leader Basil McCrea (left) and deputy leader John McCallister, as the leader of Northern Ireland's newest political party denied allegations of inappropriate sexual behaviour

Basil McCrea, Tina McKenzie and John McCallister at the launch of NI21 in June 2013

Allegations of inappropriate behaviour against the leader of Northern Ireland's newest political party are of a sexual nature, he has confirmed.

Basil McCrea vehemently denies the claims that have plunged NI21 into chaos and have cast serious doubts on whether the fledgling movement, which had promised a new brand of politics, can survive.

The Lagan Valley MLA and former senior Ulster Unionist confirmed he had emailed NI21 colleagues yesterday acknowledging that allegations of "inappropriate sexual activity" had been levelled against him.

"There obviously are social media rumours and such like, there is a specific allegation coming forward now, and now there is such a thing, then I can go and respond to it," he said.

"In these issues it's important you respond in the appropriate way and I will be looking to see how we can do that."

NI21 edged towards outright implosion as the polls closed last night after its European Parliament candidate, Tina McKenzie, dramatically resigned from the party executive and deputy leader John McCallister claimed a shock policy reversal ahead of the election was engineered by Mr McCrea to force him to quit and thwart his efforts to investigate the claims against the party chief.

Mr McCallister claimed the decision to change the party's Stormont designation from "unionist" to "other" was a provocative move choreographed by Mr McCrea to force him out because he had challenged him about the allegations about his behaviour and was overseeing an investigation into the matter.

The South Down Assembly member alleged that Mr McCrea was attempting to thwart the external human resources report he had commissioned into his conduct.

"The report comes to me, so if I was not in position, therefore, the process could be easily stopped," he said.

Mr McCrea, who has also denied the claims about the motivation behind the designation vote, admitted that NI21's future as a political party had been seriously rocked.

"It can't be denied that the damage we have suffered over the last 48 hours or slightly longer is significant and I do think it will cause challenges in the future," he said.

In an interview with BBC Radio Ulster he conceded that recent events had had a personal impact.

"I have certainly had better days in my life, there are issues undoubtedly will have to be addressed," he said.

Mr McCrea and Mr McCallister, who formed NI21 last year after they acrimoniously split from the UUP, were until recently close friends and tight political allies.

Mr McCallister claimed the decision to call the party executive together at short notice earlier this week to vote to change the party's Stormont designation was done purely to "muddy the waters" and force him to quit.

But he said there was "not a chance" he would resign.

Mrs McKenzie, who emailed party members last night informing them of her decision to quit the executive, was one of 10 candidates battling for Northern Ireland's three seats in the European Parliament.

Last night she cryptically tweeted that she would not work for "people who play dirty tricks and dirty politics".