Boris Johnson received a £20,000 interest-free loan from the company owned by top Tory strategist Sir Lynton Crosby, official documents reveal.

The former foreign secretary, who is widely thought to be planning a leadership bid, received the sum from CTF Partners, which Sir Lynton jointly owns.

The firm also donated £3,000 to Mr Johnson that was earmarked, like the loan, for "office and staffing costs", according to the latest edition of the parliamentary register of interests.

The loan was received on 20 December and must be repaid by 20 January, the document says.

MPs receive official funding for staff and office space relating to their parliamentary work, so the loan from CTF will fuel speculation that Mr Johnson is plotting a leadership bid.

The MP and Sir Lynton have remained close since Mr Johnson's time as mayor of London. The strategist ran his two successful mayoral campaigns, in 2008 and 2012, before moving on to lead the Tories' 2015 general election campaign under David Cameron.

He also worked closely with the Conservatives during the 2017 election campaign, which saw Ms May squander her Commons majority.

The Australian is a divisive figure whose campaigning methods have been hotly contested. He is an advocate of strict message discipline and a fan of the so-called "dead cat strategy", which involves making a dramatic or shocking statement to distract from another issue that is deemed to be damaging.

Mr Johnson resigned as foreign secretary last July over the government's Brexit plan and has been a vocal critic of the proposed withdrawal agreement. He is seen as highly likely to run to succeed Ms May as Conservative leader.

Police separate clashing Brexit protesters outside Parliament Show all 11 1 /11 Police separate clashing Brexit protesters outside Parliament Police separate clashing Brexit protesters outside Parliament Anti-Brexit protester Steve Bray (left) and a pro-Brexit protester argue as they demonstrate outside the Houses of Parliament Getty Images Police separate clashing Brexit protesters outside Parliament A pro-Brexit protester argues Getty Images Police separate clashing Brexit protesters outside Parliament Police surround the pro-Brexit protester after he confronted Steve Bray, a pro-European protester Getty Images Police separate clashing Brexit protesters outside Parliament A leave supporter is spoken to by a police officer as he argues with a remain supporter, Steve Bray, outside Parliament PA Police separate clashing Brexit protesters outside Parliament MPs in Parliament are to vote on Theresa May's Brexit deal next week after her December vote was called off in the face of a major defeat Getty Images Police separate clashing Brexit protesters outside Parliament A leave supporter is spoken to by a police officer PA Police separate clashing Brexit protesters outside Parliament Police look on as anti-Brexit protesters demonstrate outside the Houses of Parliament. Getty Images Police separate clashing Brexit protesters outside Parliament Police hold back a leave supporter PA Police separate clashing Brexit protesters outside Parliament A police officer speaks with anti-Brexit protester Steve Bray Getty Images Police separate clashing Brexit protesters outside Parliament Police surround a pro-Brexit protester after he confronted a pro-European protester Getty Images Police separate clashing Brexit protesters outside Parliament Conservative MP David Davies, wearing a gopro camera, speaks to anti-Brexit protester Steve Bray Getty Images

It is unclear why the loan was needed, and Mr Johnson's team declined to comment.

The Uxbridge and South Ruislip MP has a significant income, earning £275,000 a year as a columnist for the Daily Telegraph and many thousands of pounds more in royalties from his books.

In November he was paid £95,000 by a US asset management company for a two-hour speech.

He also earns £77,000 from his role as an MP.

Labour MP Owen Smith, a supporter of the Best for Britain campaign for a fresh Brexit referendum, said: “The surest indicator that we are winning the argument for a final say referendum is that the true enemies of the people – people like Johnson and Crosby – are getting ready their war chest to fight it.