No 10 spokesperson said: “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.”
Earlier, Downing Street appeared to have been blindsided by the revelation of the debt, his official spokesperson suggesting staff knew nothing about it.
The creditor and nature of the debt – revealed by Private Eye magazine, from a database of county court judgments – are unknown, but it is said to date back to last October.
Such judgments are issued in England and Wales when there has been a failure to repay a debt when “the court has formally decided that you owe the money,” according to a government website.
A bailiff “may visit your home if you do not pay your debts”, it warns – and banks and loan companies may use the information to refuse to issue credit or loans.
For that reason, the order has raised fresh questions about how Mr Johnson paid – eventually – the bill of up to £200,000 for his lavish flat refurbishment.
The prime minister’s finances have long been described as chaotic and he has complained to friends of being short of money, since taking a pay cut to enter No 10.
David Lammy, Labour’s shadow justice secretary, attacked the situation, saying: “If this was a single parent in Tottenham getting hammered by Universal Credit, screwed by austerity and kicked in the balls by the hostile environment, they would not get off as lightly.
“There can’t be one rule for the prime minister and another for everyone else.”
One legal expert suggested Mr Johnson would struggle to overturn the judgement – because so many months had passed since it was issued.
Generally, they are sent in the post, including a letter before claim, a final demand and then a claim form from the court.
It was suggested it was a “default judgment”, issued when someone has not put up a defence to the claim issued against them.
The controversy comes as the prime minister faces multiple inquiries, including by the parliamentary commissioner for standards over a luxury £15,000 Christmas holiday in the Caribbean.
The Electoral Commission has launched an investigation into who originally paid for his Downing Street flat renovations.
Mr Johnson has repeatedly failed to deny that the Conservative Party, or multiple Tory donors, paid out for the works – which he is now said to have funded.