Private Eye magazine uncovered an October 2020 county court judgment against Johnson for 535 pounds ($A973).

The court record, which names the debtor as Boris Johnson of 10 Downing St in London, doesn't disclose the identity of the creditor.

County court judgments can be issued if someone fails to respond to a court action over an alleged debt, according to a government website.

The judgment means the court has formally decided the money is owed.

The government site warns that bailiffs can be sent in if the debt isn't paid.

Johnson's office initially appeared unaware of the judgment and said it was investigating.

It later said the court order was unfounded.

"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," Downing St said in a statement.

Johnson's finances are already under scrutiny, with election watchdog the Electoral Commission investigating the funding of renovations to the prime minister's official residence.

The parliamentary ethics watchdog is also investigating who paid for Johnson's vacation on the Caribbean island of Mustique just before the coronavirus pandemic.

Australian Associated Press