The Sun's political editor reports that Mrs Leadsom phoned Mrs May to warn her of her resignation - but added the PM did not inform her own senior staff.

Downing Street has responded to Andrea Leadsom's resignation and the mounting pressure it places on Theresa May and her premiership, insisting that the PM "remains focused on delivering the Brexit people voted for".

Here's the statement in full:

“Andrea Leadsom has served with distinction and great ability as a member of the government, and the prime minister is grateful for all of her work.

"We are disappointed that she has chosen to resign, and the prime minister remains focused on delivering the Brexit people voted for.”

Labour MP Jess Phillips may not agree on policy matters with Mrs Leadsom, but she says she likes her on a personal level.

Like Mr Pursglove, Steve Double - the MP for St Austell and Newquay - has offered his support to Mrs Leadsom.

Mrs Leadsom's fellow Northamptonshire MP, Tom Pursglove, has signalled "respect and admiration" for his colleague.

Andrea Leadsom's full resignation letter in which she says she doesn't believe Theresa May's Brexit approach will "deliver on the referendum result".

Image source, Getty Images

Labour's deputy leader Tom Watson has criticised Mrs Leadsom's decision to step down on the eve of the European elections, calling it a "slap in the face" for her Tory colleagues.

He tweeted: "I accept that she may want to go but to do it the night before an election looks odd.

"It's also a slap in the face to Tory party members who are working hard to get their candidates elected tomorrow."

Labour have reacted to Andrea Leadsom's resignation, saying that it shows Theresa May's "authority is shot and her time is up".

"While the Tories are ripping themselves apart, our country is in crisis," the party's chairman, Ian Lavery, said.

"The government has made a catastrophic mess of the Brexit negotiations, our steel industry is under threat and Universal Credit is pushing people into poverty.

"For the sake of the country, Theresa May needs to go, and we need an immediate general election."

Image source, Getty Images

Andrea Leadsom - Conservative MP for South Northamptonshire - is a prominent Brexiteer who described the 2016 referendum result as "a huge opportunity for our great country".

She believes in scrapping free movement and wants free trade negotiations with the rest of the world.

Mrs Leadsom previously ran for Tory leader but withdrew, clearing the path for Mrs May to become prime minister.

She was Secretary of State for Environment, Food and Rural Affairs between July 2016 and June 2017, after which she became the leader of the House of Commons.

As Commons Leader, she was in charge of organising government business.

Closing her resignation letter, the former environment secretary paid tribute to the "integrity, resolution and determination" Mrs May has shown as PM.

She concluded: "No one has wanted you to succeed more than I have, but I do now urge you to make the right decisions in the interests of the country, this government and our party."

Image source, Reuters

Explaining her decision to resign on the eve of the European Parliament elections, Mrs Leadsom said: "I know there are important elections tomorrow, and many Conservatives have worked hard to support our excellent candidates.

"I considered carefully the timing of this decision, but I cannot fulfil my duty as Leader of the House tomorrow, to announce a Bill with new elements that I fundamentally oppose."

In a letter to the PM - posted on Twitter - Mrs Leadsom explained she had chosen to resign because she no longer believed the government's approach to Brexit "will deliver on the referendum result".

Outlining her reasons, she said:

1. I do not believe that we will be a truly sovereign United Kingdom through the deal that is now proposed.

2. I have always maintained that a second referendum would be dangerously divisive, and I do not support the government willingly facilitating such a concession. It would risk undermining the Union, which is something I passionately want to see strengthened.

3. There has been such a breakdown of government processes that recent Brexit-related legislative proposals have not been properly scrutinised or approved by cabinet members.

4.The tolerance to those in cabinet who have advocated policies contrary to the government's position has led to a complete breakdown of collective responsibility.

Andrea Leadsom, the leader of the House of Commons, has announced her resignation.

BMG Research has carried out a poll for the Independent news website - taken between 20 May and 22 May - but there has been little movement for the Conservatives, despite a somewhat chaotic couple of days.

Image source, AFP

After this afternoon's 1922 Committee meeting, Sir Graham Brady told reporters: "I will be meeting with the prime minister on Friday following her campaigning in the European elections tomorrow and following that meeting I will be consulting with the 1922 executive."

He said the executive discussed "all sorts of things" in the meeting.