Democratic Unionist Party MP Jim Shannon is being investigated by the Independent Parliamentary Standards Authority's compliance officer over expenses claims, the watchdog has said.
The rare formal probe concerns a claim by the Strangford MP relating to travel and subsistence, IPSA said.
It added that no further details of the alleged wrongdoing will be published until it had been concluded.
Last year, Mr Shannon was the highest-claiming MP - paid a total of £205,798.23 in taxpayer-funded expenses, excluding travel.
Mr Shannon said he thought the probe related to mileage claims made by his staff, though said he was not certain that was its focus.
"If that's what it's about, and I presume it probably is - the staff mileage - (then) all the facts are there, all the mileage is there, all the details are there, all the information is there - there is nothing to worry about," he said.
"There is absolutely nothing whatsoever to worry about - it's all there, it's all in black and white and it will all be shown," he said.
The watchdog's compliance officer Peter Davis will head up the investigation.
Mr Davis has faced criticism for settling almost all his cases without any publicity - despite promising to disclose the outcome of all investigations.