Iain Duncan Smith has strongly rejected "malicious" allegations of impropriety regarding the running of his private office.

The comments came after it emerged that the Conservative leader is set to be investigated by parliamentary standards commissioner Sir Philip Mawer for employing his wife, Betsy, as an aide.

Journalist Michael Crick made headlines on Sunday by pledging to send his findings to the standards commissioner.

"I believe that Sir Philip is the only man who can get to the bottom of this matter," Crick wrote in the Sunday Telegraph.

"It is in the interests of good government and, if the claims turn out to be unfounded, it should also be in the interests of Iain Duncan Smith."

But Duncan Smith later issued a statement saying he had already decided to let the standards commissioner review the matter to end any suggestion of impropriety.

"I completely and absolutely rebut any allegation about wrong-doing," he said.

However Vanessa Gearson, who ran the Tory leader's office from September 2002 to August 2003, is reported to have refused to corroborate Duncan Smith's version of events.