CONSERVATIVE leader Iain Duncan Smith faces a further investigation into allegations that he improperly employed his wife.

Parliamentary Standards Commissioner Sir Philip Mawer's decision will come as a blow to Mr Duncan Smith.

The Tory leader submitted a 40-page dossier to Sir Philip in an effort to prove there was no case to answer.

But Sir Philip said in a statement: "The nature of the information given to me - some of which is from anonymous sources - makes it necessary."

He said: "The fact that I am making further inquiries does not imply that I regard the allegations against Mr Duncan Smith as substantiated, simply that I need additional information in order to be able to evaluate them properly."

Sir Philip also challenged Mr Duncan Smith's enemies to "put up or shut up".

He said: "People who want to make serious allegations against a Member of Parliament or, indeed, anybody else, have got to be prepared essentially to put up or shut up."

His words will be welcomed by Mr Duncan Smith, who hit out at the "cowards lurking in the shadows".

"These cowards have gone after my wife as a way of getting after me and I think it's intolerable. I love my wife deeply," he said.

"Anybody who wants to come after me - come after me but don't go after my wife.

"I will never ever use my wife as a tool in my political life."

The Tory leader added: "I have asked for this inquiry, I'm pleased he's going to do it, and I believe I'm going to be exonerated by it."

Sir Philip's probe was sparked by a complaint from TV reporter Michael Crick.

Mr Crick said last night: "Many people were disgusted with the way in which his wife was apparently on the parliamentary payroll but, as far as they could see, was doing no work.

"My hunch is there will be a change of leadership."

Any suggestion that Mr Duncan Smith did act improperly would be seized on by opponents attempting to unseat him.

But Sir Philip's investigation comes as a blow to the Tory leader regardless of its findings.

He is already fighting a plot to oust him that has dominated the headlines since surfacing at the Tory conference.

The allegations from Mr Crick are seen as part of that conspiracy by Mr Duncan Smith's supporters.

The allegations were due to be screened ahead of last week's conference but the report was pulled by the BBC.

Instead, Mr Crick handed the findings of his five-month investigation into Mrs Duncan Smith's role to Sir Philip on Monday.

Mr Duncan Smith submitted his own 40-page dossier in an attempt to show there was no case to answer.

Sir Philip will present his findings to the Labour-dominated Commons Standards and Privileges Committee, which could then re-examine the evidence.

They could be inconclusive unless Mr Crick's sources are prepared to speak out.

However, those attempting to unseat Mr Duncan Smith may calculate they have done enough without risking their future careers by going public.

Mr Crick said last night his anonymous sources were ready to talk to the commissioner without their names coming out.

"The processes have been devised whereby this can be done," he told Channel 4 News. "I have not checked this with him, but I understand he is willing to talk privately."