The row over the court judgment against Boris Johnson for an unpaid £535 debt has taken a bizarre twist, after it emerged it was for a claim of defamation.

Yvonne Hobbs is reported to have launched multiple claims against the prime minister and public institutions – often sending copies of her complaints to the Queen, the BBC and Parliament.

The information has raised questions about how the claim was approved by the online small claims court, when defamation cases are usually dealt with by senior judges in the High Court.

The county court judgement – made six months ago – had appeared to be a fresh embarrassment for Mr Johnson has he faces multiple inquiries by standards’ watchdogs.

The Electoral Commission is investigating into who originally paid for his flat renovations – while the parliamentary commissioner for standards is probing a luxury £15,000 Christmas holiday in the Caribbean.

But Downing Street is confident that an application to strike out the debt judgement will be successful, describing it as “totally without merit.”

No 10 was blindsided by the story – revealed by Private Eye magazine – but quickly made clear it was nothing to do with the controversy over his lavish flat makeover.

Legal experts suggested it was a “default judgment”, issued when someone has not put up a defence to the claim issued against them.

Generally, they are sent in the post, with three different correspondence – including a letter before claim, a final demand and then a claim form from the court.

But it appears the correspondence was sent to 10 Downing Street rather than any property Mr Johnson owns, or 11 Downing Street, which is where his flat is.

A file shows the claim was made by an Yvonne Hobbs against “The Rt Hon Boris Johnson” the reasons cited being “defamation” and “committed repeated defamation”.

Under the small claims court procedure, anyone can make an official demand for another person or a business to pay them money owed, paying as little as £25 for an online claim.

A No 10 spokesman 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.”