Nearly 200 business bosses have signed a letter backing the Prime Minister's campaign to keep Britain in the European Union.

The letter - which was sent to the Times newspaper - warns quitting the EU would "put the economy at risk".

Who signed it?

Signatories including more than 35 chairmen and chief executives of FTSE 100 companies - Britain's most valuable firms.

The chiefs of Asda, BT, Marks & Spencer, Kingfisher and Vodafone are amongst them.

It was previously reported that as many as 50 of the FTSE 100 chiefs would be signing.

Notable absences from the FTSE 100 are Tesco, Sainsbury's and Barclays.

Among those who have signed are Tory donors and figures who have accepted government roles under Mr Cameron's premiership, according to The Times.

What does it say?

The letter warns of the risks to the UK economy if the majority of voters decide to leave the EU in the referendum which is planned for June this year.

It claims the businesses led by the signatories "represent every sector and region" of the UK economy and employ between them "more than one million people."

Conservative Party split

The move comes after London Mayor Boris Johnson and five Cabinet ministers declared their support for Brexit over the weekend. An ITV News poll found 120 Conservative MPs are now backing Brexit.

Roland Rudd, treasurer of Britain Stronger in Europe, which organised the letter with No 10's support, told the Times: "This is the single biggest number of business leaders who have been willing to support staying in a reformed EU.

"What is also striking is the number who have done so on behalf of their companies as well as in a personal capacity."

David Cameron earlier defended using a Downing Street civil servant to lobby businesses to support the pro-EU campaign after MPs heard the letter was about to be published.

"The Government's view is that we should remain in a reformed European Union and the civil service is able to support the Government in that role," he said.