Boris Johnson is being investigated for a possible breach of Commons rules over a luxury £15,000 Christmas holiday in the Caribbean.
The parliamentary commissioner for standards has powers that can trigger suspensions of MPs – or even by-elections – if serious breaches have occurred.
The list of MPs under investigation had been kept under wraps during the campaign for the May elections, but has now been revealed by Kathryn Stone, the independent commissioner.
Her list reveals she is looking into the mystery of who funded the prime minister’s luxury Caribbean holiday on the island of Mustique, during Christmas 2019.
At the time, Mr Johnson claimed the £15,000 cost of the villa he stayed in was paid for by David Ross, a Tory party donor and former deputy chairman of Carphone Warehouse, who owns a property on the island.
But Mr Ross then denied he had paid for the holiday and said he did not own the villa where Mr Johnson and his fiancée, Carrie Symonds, stayed.
However, the commissioner does not appear to be acting on a complaint submitted by the Labour MP Margaret Hodge, over who originally paid for Mr Johnson’s expensive flat renovations.
Mr Johnson has repeatedly failed to deny that the Conservative Party, or multiple Tory donors, paid out for works that could have cost up to £200,000.
And the Electoral Commission also launched an investigation, saying “an offence or offences may have occurred”.
It is the first time a serving prime minister has been investigated by the commissioner, who is responsible for regulating MPs’ conduct and adherence to parliamentary rules.
Ms Stone is also investigating seven other Conservative MPs – former cabinet ministers Owen Paterson and Theresa Villiers, plus Adam Holloway, Bob Stewart, Natalie Elphicke, Roger Gale and Jonathan Gullis.
Claudia Webbe, elected as a Labour MP in 2019 but suspended from the party in September 2020 after being charged with harassment of a woman, is also under investigation.
Labour seized on the probe into the prime minister’s holiday, deputy leader Angela Rayner saying: “Another day, another investigation into Boris Johnson for more sleaze and dodgy dealings.
“The public have a right to know who paid for Boris Johnson’s luxury Caribbean holiday and the renovation of his flat. Most importantly, we need to know what these donors were promised or expected in return for their generosity.”
However, Labour’s attempts to puncture Tory support at last week’s elections by focusing on “sleaze” are acknowledged to have failed by all sides.
The announcement says Mr Johnson is being investigated under “Paragraph 14 of the Code of Conduct” relating to “Registration of interest under Category 4 of the Guide to the rules [Visits outside the UK] in 2020”.
A spokesman for Mr Ross said: “Mr Ross facilitated accommodation for Mr Johnson on Mustique valued at £15,000.
“Therefore this is a benefit in kind from Mr Ross to Mr Johnson, and Mr Johnson’s declaration to the House of Commons is correct.”
Mr Johnson’s spokesman said: “The prime minister transparently declared the benefit in kind in the Commons register of interests.
“Clearly the rules are set out and it’s important that everyone abides by them, as the prime minister has done throughout.”