Boris Johnson has launched a stinging attack on “karaoke” Sue Gray and her pal Sir Keir Starmer.

The former prime minister whose tenure in Downing Street was partly brought to an end by Sue Gray’s partygate report accuses the former top civil servant and Sir Keir of doing nothing to cut the cost of government while “capitulating” to union pay demands.

Mr Johnson writes of Sir Keir and Ms Gray in his latest column for the Daily Mail: “They have bowed down to the public sector unions, and abandoned all attempts to cut the cost of government – which in its complexity and luxury is starting to resemble the later Roman empire.

“Take the so-called ‘Cabinet Office’, that mysterious entity at the heart of Whitehall, where people such as Sue Gray – former senior civil servant turned Starmer’s chief of staff – used to spend her time on the karaoke machine.

“When Margaret Thatcher left No 10 in 1990, it numbered 600 people. When Tony Blair left office in 2007, it had reached 1,000. There are now about 8,000 of them.”

Mr Johnson asked: “What are they all doing, and what is Starmer doing about it? Nothing, except of course giving everyone the right to work from home and not to be disturbed at the weekend by their boss.”

His claim Ms Gray spent time on a karaoke machine at the Cabinet Office appears to recall an image from before the Covid pandemic where the former civil servant can be seen holding a microphone in front of a screen with lyrics from the Grease track You’re the One that I Want on it.

GB News first reported the image in June 2022, suggesting it illustrated a culture within Whitehall at the time. The broadcaster reported the same karaoke machine is mentioned in Ms Gray’s report into lockdown parties in Downing Street. There was no suggestion of wrongdoing on Ms Gray’s part.

The Cabinet Office has been approached for comment. Ms Gray has been approached via the Cabinet Office.

On its website, the government department said that as of 2021-22 it employed about 2,050 staff, with some 700 working for the Crown Commercial Service. Other sources put the total above 10,000.

It describes its purpose as supporting the prime minister and ensuring the “effective” operation of government besides taking the lead in “critical” policy areas.

In his lengthy diatribe, Mr Johnson accuses the Labour government of junking Conservative plans to reduce the government headcount by 66,000 and clobbering businesses and Brits to pay for a growing state.

The former prime minister also sounds the alarm about a “brain drain” and exodus from the UK as rich entrepreneurs and businesspeople head overseas to avoid paying higher taxes under the Labour Government.

His comments amid mounting speculation as to how Chancellor Rachel Reeves will seek to plug a £22billion black hole in the country’s finances identified by the government.

During its election campaign, Labour ruled out raising taxes on “working people”, which led some analysts to suggest the Government may seek to fill its coffers through hikes in inheritance tax, capital gains tax or council tax.