The county court issued the judgment against the Prime Minister’s name last October, according to Private Eye magazine.
The record lists the Prime Minister’s name as “Boris Johnson” and gives his address as 10 Downing Street. It shows the debt remains “unsatisfied”.
The court judgment was issued around the same time Mr Johnson was facing questions over the cost of renovations to his Downing Street flat.
Details of the debt are not known. The Standard has approached No10 for comment.
Angela Rayner, Labour’s Deputy Leader, commented: “Another day, another report of deeply concerning irregularities about the renovation of Boris Johnson’s flat.
“This is not about Boris Johnson’s personal finances, the record speaks for itself that he has already broken the rules on declaring his financial interests, and he is already under investigation regarding potentially illegal wrongdoing.
“The issue of debt when it comes to the Prime Minister is whatever debt of gratitude Boris Johnson owes to the Tory donor who paid to renovate his flat, and what this donor or donors were promised or expected in return for their generosity.”
The Prime Minister’s official spokesman told a briefing of journalists: “I’ve seen that report. We are looking into this issue.
“But I don’t have anything further for you on that at the moment. We are looking into that report currently.
“I can confirm it is nothing to do with the refurbishment of the Downing Street estate where all such bills have been duly paid - either by the Government or by the Prime Minister personally.”
Asked if the PM had settled the debt, he replied: “Like I say, we’re looking into the specific issue that’s been raised this morning.”