The party said yesterday that Sir Thomas had identified “an error made by the House of Commons Fees Office in relation to the overpayment of two months’ rent for Martin McGuinness’s rental property in London in 2006”.

It said there had been no request for any of the other four Sinn Fein MPs to pay back any monies.

The fees office had lost a claim for rent and asked for a duplicate to be faxed to it in August 2006, said Sinn Fein.

The duplicate was sent and the bill was paid on September 1 and again on October 4.

The party said: “Having had this matter brought to our attention we have immediately taken steps to refund this overpayment.”

All expenses claims for Sinn Fein MPs are prepared by the party finance department and at no time was any money paid directly to Mr McGuinness, it said.

Meanwhile, Ulster Unionist MP Lady Hermon has revealed that she has been asked to repay £1,365 which Sir Thomas said was overpaid in rent during January 2007.

In a statement, the North Down MP said that she had repaid the money to the Commons: “In January 2007, my then landlord increased the rent of the London flat and he also wrote to me asking for the rent to be paid a month in advance. I forwarded the landlord’s letter to the Fees Office at that time, and so it should be on file there.

“Consequently, there was indeed a double payment of rent by the Fees Office in the month of January, but none was then paid in February.”