expenses payback bill to date of £63,250 as it emerged yesterday that a trade union is preparing to challenge an expected ban on MPs

employing their relatives.

Mr Jenkin, MP for North Essex, has urged Sir Thomas Legg, the former Whitehall mandarin conducting the investigation, to drop the demand as he insists he abided by the Commons rules at all times.

However, he added he would be willing to repay “whatever is finally recommended at the end of the process” – falling into line with David Cameron’s order to Conservative MPs to repay what is asked.

The repayment bill relates to a second-home allowance Mr Jenkin spent on renting a farmhouse from his sister-in-law from 2004.

The former shadow defence secretary insists the Commons Fees Office sanctioned the arrangement but failed to inform him when payments to relatives were outlawed in 2006.

Nonetheless, Sir Thomas has told him to repay the full five years’ rent.

Pointing out he kept “within the letter and spirit of the rules at all times”, Mr Jenkin said: “No question has been raised about my honesty, integrity or openness and I have reiterated to him that I will pay back whatever is finally recommended at the end of the process.”

Thus far, Prime Minister Gordon Brown has been among MPs facing the highest repayment bills, paying back £12,415 for cleaning and gardening, even though the claims had been within the rules at the time.

Sir Thomas’s final findings were expected to be published early next month, but it is thought this could be delayed by weeks if not months, given the number of MPs querying his findings.

Meanwhile, the report on the system of MPs’ expenses by Sir Christopher Kelly is due to be published next week. It is widely expected to recommend a ban on MPs employing their relatives.

However, Unite, which represents public sector workers, has vowed to oppose the move and will hold consultations to discuss the options open to MPs’ spouses, around 200 of whom work at Westminster.

The union plans to use employment law to prevent Sir Christopher from forcing MPs’ relatives to give up their jobs.

Yesterday, David Cameron once again sought to take the initiative over expenses by announcing every Tory peer who claims overnight expenses for attending the House of Lords will be required to identify clearly which is their main residence.

The move follows a string of controversies involving peers who claim the allowance despite having a home within a few miles of Westminster, telling House authorities they need allowances to stay overnight in London because their main address is elsewhere in the country.

The Tory leader said: “This is our step to put an end to that process, and I hope others will follow our lead.”