The expenses of DUP MP Jim Shannon accounted for a quarter of the staff mileage claimed by the entire House of Commons in 2014/15, an investigation has found.

The genuine claims were five times greater than the second highest claiming MP and 37 times greater than the average across all MPs, a parliamentary watchdog said.

Total overpayments of £13,926 will be repaid, the Strangford MP said.

Record keeping was completely inadequate and methods for the submission and collection of mileage claims were "woeful", with every staff member having a separate system, according to the Independent Parliamentary Standards Authority (Ipsa).

It praised Mr Shannon's diligence in taking on a wide range of issues but said he could more evenly distribute constituency work among Stormont and council colleagues. The report said: "The situation is unsustainable."

Due to his diligence Mr Shannon had become a single point of contact for a wide range of issues instead of acting as a conduit to more appropriate agencies.

Claims for staff constituency mileage began in January 2013 and by the end of the 2014-15 financial year his staff had travelled 121,259 miles.

Ipsa formed the opinion that the “level of commitment of staff time to driving appears to be neither practical nor plausible”.

The report said: “He accounted for 26.1 per cent of all staff constituency mileage claimed by the entire House of Commons.”

Mr Shannon's Strangford constituency has a smaller population than most in Northern Ireland and a smaller percentage of claimants for disability benefits and for unemployment related benefits, although it has a poor transport network and older population, the report said.

Staff hand-delivered mail because they had overspent Mr Shannon’s office costs expenditure budget. A sample of a hand-delivered leaflet contained a DUP logo and was not allowable under the expenses scheme.

The review said: “This situation cannot be sustained and Mr Shannon must consider the use of electronic communication to reduce both his office costs expenditure and staff constituency travel.”

A part-time member of staff had the highest mileage claims over the last two years after working considerably more than the contracted hours.

Taken in their entirety, the travel and subsistence claims made by Mr Shannon in 2014-15 were £14,289 greater than any other MP.

The veteran public representative said he and his staff worked tirelessly for the people of Strangford. “The work MPs do in serving their constituents is publicly funded, and there must be no question of any abuse or dishonesty in terms of how those funds are used.

“I am particularly glad that nothing of this sort has happened in my case and that my integrity and that of my office has been upheld.

“Ipsa have acknowledged the special circumstances elected representatives in Northern Ireland personally face in dealing with the aftermath of a whole political generation lost to the distorting effect of terrorism.

“But I want to reiterate that, although I am determined to give my constituents the very best service I can, I know that the work done to support me must be undertaken as economically and efficiently as possible.

“I will therefore be implementing, in full, the scheme recommended for the reorganisation of my office.”