* Businesses worried as May's deal attacked
* Shares in banks, retailers and builders tumble
* Fears that no-deal Brexit could freeze supply chains
By Kate Holton, Andrew MacAskill and Georgina Prodhan
LONDON, Nov 15 (Reuters) - Business leaders expressed alarm on Thursday as a draft Brexit agreement seen as the only chance of preserving some stability in UK-EU trading relations threatened to unravel, sending stock prices and the pound plunging.
Just 12 hours after British Prime Minister Theresa May announced that her cabinet had agreed to the terms of the draft agreement, Brexit minister Dominic Raab and work and pensions minister Esther McVey quit, saying they could not support it.
Their departures and those of other, junior ministers, revived the spectre for business of Britain leaving the European Union without a deal next March, and sent shares in British housebuilders, retailers and banks tumbling.
"The situation this morning saps the confidence of the City and the country," said Martin Sorrell, ex-CEO and founder of ad agency group WPP and one of Britain's best-known businessmen.
The European Union is Britain's biggest trading partner, accounting for 44 percent of UK exports and 53 percent of imports to the UK.
After 45 years of membership, industries including defence, cars and aerospace have created intricate supply chains that rely on smooth, "just-in-time" delivery of thousands of parts across the sea that divides Britain from the continent.
Business leaders fear that the country could stumble towards a no-deal Brexit where border checks block ports and fracture the supply chains that support the likes of Rolls-Royce and BAE Systems .
A senior executive at one of Britain's biggest banks said this was the most disastrous government he had ever seen.
"The rest of the world is looking at us and laughing. When I travel everyone wonders whether we have gone completely mad," he said.
"It is time to have some stability so business can get some certainty. This is what the country needs."
Industry bosses who had been briefed on the draft agreement by ministers late on Wednesday had broadly welcomed it as the best chance of a compromise that would secure a transition period and avert the chaos of no deal at all.
May's office also released statements from a number of major companies such as Diageo, the London Stock Exchange and Royal Mail welcoming the draft deal.
"Most business people ultimately are pragmatists and this is about playing the cards we have been dealt rather than wishing for a better hand," Roger Carr, chairman of BAE Systems, told BBC Radio on Thursday morning.
Iain Anderson, executive chairman of public affairs firm Cicero, which represents many finance companies, said although most executives did not like May's deal they realised it was now the only game in town.
"Business is watching with horror the resignations now taking place," he said. "Yesterday we had a plan and stability and today we do not.
"There is now no time to negotiate another deal. We thought we had stability - now we have instability writ large," he said.
The UK chief of German industrial group Siemens, which employs 15,000 people in the UK, reiterated his call to get behind the draft agreement even as senior politicians called for May to quit.
"We hope all sides keep calm, look at the facts, and move to support this draft to provide UK business with greater certainty," Juergen Maier said in an emailed statement.
Even if May survives, her chances of winning a vote in parliament to approve the draft agreement are seen as slim.
One senior executive at a FTSE 100 company was still holding out hope however that lawmakers would eventually be persuaded to vote for the deal when it comes before parliament before the end of the year.
"We're going to need the market to throw up and scare them all into voting for it," he said. The pound was down 1.5 percent against the dollar in afternoon trading.
Lawmakers across the political spectrum have said May's deal will leave Britain bound by EU rules without having any say. Many have argued it will also damage the integrity of the United Kingdom by aligning Northern Ireland with the rest of the EU in order to avoid a hard border with EU-member Ireland.
The CEO of French outdoor advertising company JCDecaux , which runs London's bus-shelter advertising and makes 10 percent of its sales in Britain, called the situation "obviously very serious".
"Today's events reinforce the uncertainties in the market," Jean-Charles Decaux told Reuters in an interview on the sidelines of an industry conference in Barcelona. (Reporting by Andrew McAskill, Sarah Young, Kate Holton, Mathieu Rosemain and Georgina Prodhan Writing by Georgina Prodhan Editing by Keith Weir)