Sir Mick Davis has quit as chief executive and treasurer of the Conservative Party, warning the party risks the "very bad outcome" of putting Jeremy Corbyn in Downing Street if it is not "properly resourced and financed".

Sir Mick, a former Jewish Leadership Council chair, wrote to the party's biggest donors on Wednesday, saying "one of my great bugbears has been the lack of unity in our party".

He said Boris Johnson, who was elected leader on Tuesday and will become prime minister on Wednesday afternoon, "should be free to choose a team at CCHQ".

"Good politics is not a cheap exercise and if we are not properly resourced and financed we will risk a very bad outcome – Jeremy Corbyn in Number 10," he said.

"Our new leader and Prime Minister can only be effective if a strong and unified party stands behind him. One of the key roles of CCHQ is to facilitate that outcome but I am asking you our donors, upon whom we depend, to embrace that call for unity as well.

"There is today no room for bystanders – my successor will need your help and I hope that you will step forward right now and send a powerful message of unity, purpose and commitment."

In April, The Times reported Sir Mick was using his own money to fund the Tory campaigning in the European Parliament elections as donors deserted the party.

His task was to bolster party funds at a time when many of the party’s traditional donors were deserting them – mainly over fears that its Brexit position would be disastrous for business.

Sir Mick’s efforts are believed to have met with only limited success – with political events outside of his control playing havoc to his efforts.

Sir Mick took over the party after the dire 2017 general election result.

In a signal that Theresa May recognised the challenges facing the party, she said on Sir Mick’s appointment: “The party will benefit from his entrepreneurial and leadership skills as he works with the team at Conservative Campaign Headquarters to make the improvements that we need, now and for the future.”

South African-born Sir Mick, who stood down from the JLC in March 2017, was appointed Tory Party treasurer in February 2016.

He has long been a donor to the party and received a knighthood for his work chairing the former Prime Minister David Cameron's Holocaust Commission in 2015.

Sir Mick, said to have earned a reported £75million when he left mining firm Xstrata as it was taken over in 2013, is also a former UJIA leader.

Sir Mick Davis' letter to Tory donors in full

Dear 

I have submitted my resignation as CEO, Treasurer and member of the Board of the Conservative Party with immediate effect.

I have so much enjoyed my time as Treasurer and CEO, but circumstances change and I think that our new leader, Boris Johnson, should be free to choose a team at CCHQ who can work together to ensure that this organisation realises its central purpose – a clear and decisive victory at the next General Election.

Our business plan has been carefully designed to achieve that goal but it needs to be properly executed and this is where you come in. Your support for the Party and for me is deeply appreciated. But our plan needs to be properly financed on an ongoing basis and as our most important donors I am writing to ask you to get behind our new leader, Boris Johnson, and make sure CCHQ can deliver. Good politics is not a cheap exercise and if we are not properly resourced and financed we will risk a very bad outcome – Jeremy Corbyn in Number 10.

You all know that one of my great bugbears has been the lack of unity in our party. Divided parties do not win elections. Brexit with all its complexities and associated dogmas provides the unfortunate backdrop for division but if we are to effectively stand against the frightening threat of a Corbyn led Labour Government we all must find a basis of coming together.

Our new leader and Prime Minister can only be effective if a strong and unified party stands behind him. One of the key roles of CCHQ is to facilitate that outcome but I am asking you our donors, upon whom we depend, to embrace that call for unity as well. There is today no room for bystanders – my successor will need your help and I hope that you will step forward right now and send a powerful message of unity, purpose and commitment.

With my warm regards and grateful thanks to you all.

Mick