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Uncertainty surrounds the leadership of National's Don Brash who is on leave after one of his MPs confronted him about having an affair.

Deputy leader Gerry Brownlee yesterday rejected a suggestion that Dr Brash's troubles raised any issues of leadership.

But many caucus members spoken to said there was internal speculation about Dr Brash's future over his own motivation and how he now handles the matter.

One MP said some of his colleagues were saying the allegations undercut what many believed was Dr Brash's selling point - honesty.

"It's already becoming a target of opportunism," one said, referring to the leadership issue.

"People are saying he can't survive this."

Dr Brash, 65, issued a statement from Auckland yesterday saying he and his wife, Je Lan Brash, had been having some difficulties.

"Je Lan and I are working through these difficulties and we are both committed to making our marriage work.

"As people will understand, this is a very difficult time for me and my family."

The Independent newspaper reported yesterday that Dr Brash was questioned about his private life at a party caucus meeting on Tuesday, though it did not identify Rakaia MP Brian Connell, who had asked if it was true he was having an affair.

Mr Connell, in front of the caucus, said if he was lying to his wife he would be unfit to lead the party.

Dr Brash is understood to have neither confirmed nor denied the accusation but said he had not lied to the public.

It is believed the question raised in the caucus concerns Dr Brash's relationship with Diane Foreman, a married businesswoman.

She is chief executive of the Emerald Group investment company and deputy chairwoman of the Business Roundtable.

It is believed the pair had a relationship that began before Dr Brash entered Parliament but it is not known when or if the relationship ended.

Mrs Foreman had "nothing to say" when contacted by the Herald last night, but her husband Bill - who said he would be joining her in Australia this week - dismissed the allegations as "rubbish".

In a brief interview with TV3, she asked why they were talking to her.

TV3 said: There's speculation around Parliament that he's been seeing you in a relationship.

Mrs Foreman: Look, I'm a private person. I don't have to answer questions about my life. It's Dr Brash's life you're interested in. Give him a call.

TV3: Is it true you've been seeing Dr Brash?

Mrs Foreman: I find this totally insulting.

She then hung up.

The caucus showdown is linked to the tumultuous few weeks in Parliament over claims of unlawful election spending and corruption in Labour.

It is thought that when Labour frontbencher Trevor Mallard last week made a retaliatory threat to reveal the private lives of MPs, he was referring to Dr Brash.

Mr Mallard's threat followed interjections by him and David Benson-Pope in the House the previous day.

When Dr Brash rose to ask a question, Mr Mallard said "how's Diane this week?" and when he mentioned the "Phillip Field affair", Mr Mallard said "speaking of affairs ..."

Prime Minister Helen Clark said yesterday that she did not want to comment on personal matters but distanced her party from any responsibility for the matter being made public.

- Additional reporting David Eames