As Theresa May was walking back into No10 after announcing her resignation, attention was already turning to those plotting to succeed her.

The frontrunner to replace her in Downing Street is Boris Johnson, the former foreign secretary, whose fundraising efforts suggest he has been planning his leadership campaign for some time.

According to the register of MPs' financial interests, Mr Johnson has raised more than £800,000 in the last year alone. This is on top of his £79,500 salary as a MP, and income from two rental properties.

While he may be hindered by questions about his competence and strong opposition from some centre-left Conservative MPs, it appears that money will be no barrier to Mr Johnson realising his long-held desire to be prime minister.

The Uxbridge and South Ruislip MP's biggest source of income is from speaking engagements, which have earned him more than £360,000 in the last six months alone.

In November, he was paid almost £95,000 for a two-hour speech at an asset management company in New York. A month later, he received just under £29,000 for speaking at events company KNect365 in London.

Further events this year have seen him bank £51,000 for a speech at an events and training company in Dublin, more than £40,000 from Citigroup bank in London and £25,000 from a private bank in Geneva.

He also earned a hefty £123,000 from Indian company Living Media for a three-hour speech in Delhi.

Mr Johnson's announcement that he was running to be Conservative leader came earlier this month during a speech at an insurance industry event in Manchester. And on the day that Ms May confirmed the date of her resignation, he was speaking at an economic conference in Switzerland.

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In addition to his speaking fees, Mr Johnson has also registered a number of hefty private donations - £154,000 in total since last October.

This includes £45,000 from construction company JCB, whose owner, Lord Anthony Bamford, is a vocal Brexiteer.

The MP has also received £50,000 from hedge fund owner Jon Wood, and £36,000 from another hedge fund manager, Johan Christofferson.

His donations include a £3,000 gift and a £20,000 interest-free loan from CTF Partners, the political strategy firm partly run by Sir Lynton Crosby, who masterminded Mr Johnson's London mayoral elections in 2008 and 2012 and the Conservatives' general election campaigns in 2015 and 2017. Sir Lynton is said to be giving Mr Johnson informal advice on his bid to become prime minister.

The former foreign secretary also makes a sizeable income from royalties on his books, including almost £30,000 in the last year.

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His other writing, mostly in the form of newspaper articles, is another lucrative source of income.

He is paid £275,000 a year to write a weekly column for The Telegraph - equating to almost £5,300 per article. Given Mr Johnson estimates that he spends only ten hours a month on these articles, they are unlikely to take up too much of his campaigning time.

The MP has also been paid more than £3,000 for other articles in recent months, including by The Spectator, the Daily Mail and the Washington Post.