Boris Johnson will seek to quash a county court judgment for an unpaid debt of £535, Downing Street said on Wednesday, describing the claim as “totally without merit”.

The judgment against the prime minister was issued on October 26 last year by the online county court, Private Eye magazine revealed. It shows an “unsatisfied record” — meaning Johnson still owes the money.

Number 10 said the court claim was unfounded and would be challenged. “An application will be made for an order to set aside the default judgment, to strike out the claim and for a declaration that the claim is totally without merit,” a spokesperson said.

Downing Street also said the claim had nothing to do with the refurbishment of Johnson’s private residence above 11 Downing Street last year that cost tens of thousands of pounds. “All bills have been paid by the government or the prime minister, personally.”

One senior government insider said the county court judgment was a “vexatious claim” and “without merit”. The official added: “There’s no substantive evidence behind this at all.”

It is unknown who made the court order or whether the claim was ever served on Johnson. The fact that the issue reached the court would suggest that the prime minister had been asked previously to repay the debt.

David Greene, senior partner at law firm Edwin Coe, said that any defendant in the county court can apply to have a default judgment set aside as of right if the claim had not been served properly in the first place — for example if it had been sent to an old address and the defendant was unaware of the lawsuit.

He said that if a claim had been properly served, then a defendant had to show they had a proper defence if they wanted to apply for the ruling to be set aside. This might involve demonstrating that an outstanding debt had been paid or was not due in a money claim, for example.

“The court has some discretion and this sort of application can be made on the papers — although there can be a hearing,” he said.

County court judgments can stay on an individual’s credit record for six years, impairing their ability to borrow from banks, and can be used as a basis for sending a bailiff to collect the owed money.

The news comes as Johnson faces continued questions over how he paid for the refurbishment of the Downing Street flat.

Leaked emails showed that Tory donor Lord David Brownlow gave £58,000 to cover payments “already made” by the Conservative party towards the project, and wanted the donation to be attributed to a “soon-to-be-formed Downing Street trust”.

The Electoral Commission, which monitors political party spending, has begun an inquiry into the apartment, saying there were “reasonable grounds” to suspect an offence.

Johnson has a history of carelessness with money, having previously had to apologise to MPs for the late declaration of more than £52,000 of income from a newspaper column and books.

In 2008, the then editor of GQ, Dylan Jones, said: “I have employed Boris Johnson for eight years as GQ’s motoring correspondent, and even though he’s probably cost the company £5,000 in parking tickets, I wouldn’t have him any other way.”

When Johnson was editor of The Spectator magazine, interns complained that he never repaid them for rounds of coffee. Meanwhile, Jennifer Arcuri, his former girlfriend, said he had to borrow £3.10 from her when they went on a date.