The DUP MP Jim Shannon has been ordered to repay almost £14,000 in mileage claims.
An investigation into the expenses of the MP was conducted by the Independent Parliamentary Standards Authority (IPSA) over the "unusually high" mileage claims.
It released its findings on Thursday and said £13,925.86 must be repaid by the MP which he has agreed to do immediately.
It found breaches of the MPs’ Scheme of Business Costs and Expenses by his constituency workers for claiming mileage which was thought to be significantly above that which would normally be expected
Staff members claimed over 115,000 miles in the 2012/13 and 2013/14 financial years totalling £30,000 in claims.
Mr Shannon's claims were five times greater than the second highest claiming MP and 37 times greater than the average across all MPs, the report said. His office accounted for 26.1% of all staff constituency mileage claimed by the entire House of Commons.
From these claims, IPSA formed the opinion that the “level of commitment of staff time to driving appears to be neither practical nor plausible”.
However, the investigating compliance officer concluded that there was nothing to suggest there was "dishonesty" in the claims or that they were "not genuine".
The investigator spent two days in the constituency office and noted that telephones rang incessantly and there was a constant flow of people giving the impression of a "hospital casualty department".
Among the issues raised were staff carrying out deliveries from a food bank, travel outside constituency boundaries and working for people in areas outside Strangford. As Mr Shannon had spent all his stationery allowance staff would hand deliver letters and leaflets.
There was also confusion over the work conducted by one staff member and whether that fell under their responsibilities as a councillor or constituency worker for the MP. Mr Shannon said care will be taken in the future to avoid conflicts of interest.
Records were deemed to be "completely inadequate". In one instance staff recorded mileage travel on a scrap of paper.
A widely distributed DUP-branded leaflet was determined to be a "news letter" and not allowed under expenses rules.
The investigating officer, noted the hard work and dedication of the worker in his report and said Mr Shannon should refer work on to councillors and MLAs where appropriate and use electronic communication to reduce paperwork.
He added: "Breaches of the Scheme have become increasingly widespread amongst constituency office staff, when claiming constituency mileage, during a period of over three years. As the number of staff members claiming mileage has increased, so has the value of the mileage being incorrectly claimed.
"As mileage claims have increased, so has the disparity between the overall Travel and Subsistence Expenditure claimed by Mr Shannon and every other MP. Taken in their entirety the travel and subsistence claims made by Mr Shannon in 2014-15 were £14,289 greater than any other MP.
During his investigation, the compliance officer "rigorously scrutinised" all staff constituency mileage claims and cross referenced these with the "limited mileage" records provided by Mr Shannon’s constituency office manager.
"The current situation is unsustainable," the compliance officer wrote in his report, "and Mr Shannon must engage with IPSA with a view to conducting a fundamental review of his constituency office management and staff deployment.
"He is already in ‘offsetting’ within his Staffing Expenditure budget and with the failure of his contingency application for 2015-16, will shortly overspend on his Office Costs Expenditure budget. "
The Compliance officer also recommended capping mileage spending for the Strangford representative to £10,000, with a review in future years. He also recommended a cap for all MPs.
In his response to the investigation, Mr Shannon said he could not "say no" to a constituent, no matter what issue they may have, whether it be for responsibilities which may rest with council or Assembly. Community representatives also told the investigator that they would bypass the councillor or the MLA in favour of the MP.
He also said the "democratic deficit" between 1972 to 2007 under direct rule, the local MP was regarded as the main port of call for any issue of government.
Mr Shannon - and the DUP - also said there should be recognition of the political legacy in Northern Ireland and the "dark years" with a regional variation in the rules similar to the London allowance for the higher cost of living in the capital.
"Clearly, the fact that such an arrangement lasted for over 30 years has created a culture in Northern Ireland that regards the role of an MP as being much more extensive that would be the case in other parts of the UK," he said.
"Personally, I have known constituents to become very offended when it is suggested that their problem be referred to a local MLA or Councillor as they feel the MP is the 'senior person' in the constituency and jolly well ought to be dealing with their problems, even if they are not related to parliament."
He said travel outside constituency boundaries was necessary as he was recognised as a parliamentarian in Northern Ireland who may be able to assist the 100,000 members or veterans of the defence forces especially as some had a Member of Parliament who did not take their seat in the Commons.
Mr Shannon said he will implement fully the recommendations of the watchdog.
He added: "My staff and I work tirelessly for our constituents and this has been acknowledged by the Compliance Officer in his report and in the meetings held with the MP. It is most unfortunate that the industriousness of the MP and his staff has resulted in staff mileage claims that are well above the average for Members and have given rise to this investigation. We welcome the conclusion reached by the compliance officer that 'all the available evidence leads the compliance officer to conclude that the mileage claims are genuine, insofar as there has been no dishonesty'.
"This is important for my integrity and reputation and that of my staff."