BESIEGED Tory leader Iain Duncan Smith last night faced a Commons standards investigation as he battled to head off a plot to overthrow him.

Mr Duncan Smith insisted he had done nothing wrong in employing his wife Betsy during the first year of his leadership and threatened to sue over newspaper allegations.

Journalist Michael Crick, who made the claims, last night said he was taking the issue to the Commons Standards Commissioner.

Mr Duncan Smith said he had already intended to ask the commissioner, Sir Philip Mawer, to investigate "so that we can see the full facts and clear this up once and for all.

"I completely and absolutely rebut any allegation about wrong-doing," he said. "These allegations are without foundation and are malicious and I will be contacting the commissioner first thing on Monday morning.

"I have also instructed my lawyers to review the Press reports with a view to taking legal action."

However Vanessa Gearson, who used to run Mr Duncan Smith's office, has also consulted lawyers over the row.

Mr Crick says an e-mail from Mrs Gearson, who is now deputy director of Conservative Central Office, is among the evidence he collected during a five month investigation into Mrs Duncan Smith's role.

Her solicitors, Healys, said in a statement: "In light of recent developments and growing press speculation concerning the employment of Betsy Duncan Smith, our client has sought independent legal advice.

"A letter has been sent on her behalf to Iain Duncan Smith clarifying the facts and placing on record her determination to protect her integrity and that of the Conservative Party."

Party chairman Theresa May later said: "We will be expecting a full explanation of her recent actions."

Mr Crick planned to make his allegations in a TV report due to be screened ahead of last week's Tory Party conference in Blackpool. That was pulled by the BBC and instead he is handing his evidence to Sir Philip's office today.

The journalist said last night: "The best thing for Iain Duncan Smith to do would be to get his colleagues and staff to come forward and back him up. Obviously it will take the Standards Commissioner a long time to investigate and, while he does, doubts will remain.

"It is now 11 days since his lawyers said there were people who would back him up. But now one of those a reasonable person might have expected to come forward - Vanessa Gearson - has consulted her lawyers. I thought Mr Duncan Smith said he was going to sue - is he or not? This is all very strange."

Friends of Mr Duncan Smith are adamant that colleagues will back him. Tory MP Nick Gibb suggested the allegations would damage attempts to see off a coup but still predicted it would fail.

"Until the allegations in the newspapers this morning I did not think there was going to be a change of leader and there probably still will not be," he told BBC1's Breakfast with Frost.

Shadow Home Secretary Oliver Letwin said: "I always take Iain Duncan Smith completely at his word because I have worked with him for years and I have found him to be scrupulously honest."

Meanwhile, speculation continued over what might happen once MPs return to Westminster this week.

Up to 15 of the 25 Tory MPs needed to force a vote of confidence are already said to have come forward. Plotters are being threatened with written warnings to their constituency associations.