* Bosses of 36 of FTSE 100 bosses caution over 'Brexit'
* BT, Marks & Spencer, BP says exit would cost jobs
* Tesco, Sainsbury's CEOs refrain from weighing in
* For full multimedia coverage please click: cpurl://apps.cp./cms/?pageId=brexit (Adds further comments, IOD data)
By Kate Holton and Sarah Young
LONDON, Feb 23 (Reuters) - The bosses at more than a thirdof Britain's biggest companies including oil giants Shell and BPand its largest telecoms group said leaving the European Unionwould put the economy at risk, backing Prime Minister DavidCameron's call to stay in the bloc.
The business support provided a boost for Cameron in hisbattle to win over a divided public and confront the manyeurosceptics in his own party before Britain holds a referendumon whether to stay in the EU on June 23.
It came the day after the pound posted its biggest one-dayloss in almost six years on concerns Britain might vote to leavethe 28-member bloc.
Nearly 200 companies, among them 36 FTSE 100 firms,including telecoms group BT, retailers Marks & Spencer and Asda, and oil firms Shell and BP, signed the letter which put forward the economic meritsof EU membership.
"Business needs unrestricted access to the European marketof 500 million people in order to continue to grow, invest andcreate jobs," said the letter, published in the Times newspaperon Tuesday. "We believe that leaving the EU would deterinvestment, threaten jobs and put the economy at risk."
The high-profile backing for Cameron follows a tough fewdays since he agreed new British terms with EU leaders at asummit in Brussels last week when deep divisions in hisConservative party over Europe rose to the surface.
He suffered a major blow on Sunday when London mayor BorisJohnson, one of Britain's most popular politicians, said hewould campaign for a British exit, and on Monday he was forcedto mount a defence of his deal in parliament in the face ofcriticism from some of his own lawmakers.
Johnson, who as mayor presides over one of the world'spre-eminent financial centres, dismissed the significance of theletter from the business leaders.
SPREADING ALARM
"There will be people who try to spread alarm, anxiety," hetold reporters, saying the same people had been wrong overBritain's decision not to join the euro and to exit the EuropeanExchange Rate Mechanism 20 years earlier.
Other commentators said the potential boost for Cameron waslimited given that two-thirds of top bosses had not signed up,including those from its two biggest retailers.
Tesco, Britain's biggest private employer with310,000 staff, said the referendum was a decision for the peopleof Britain and that its focus remained on serving its customers,while Sainsbury's, the country's second biggestsupermarket, said it was an apolitical organisation.
"I suspect that the numbers (of FTSE 100 bosses) givingsupport to remain publicly have been a disappointment," saidAndrew Hawkins, founder of polling firm ComRes.
In another survey published by the Institute of Directors onMonday, six in 10 of nearly 700 of its members polled said theyplanned to vote to stay in the EU.
SCOTTISH EXPERIENCE
"I can tell you this, if the leave campaign could produce 35business leaders of this sort of stature, they'd be over themoon," Cameron said of the business leaders' letter during avisit to Slough, west of London.
"This is simply people running some of the largestbusinesses in our country that employ over a million peoplebetween them, saying this has real consequences for ourcountry."
Economic concerns played a role in the Scottish independencereferendum in 2014, when in the weeks ahead of the vote, bigcompanies warned about the impact of leaving the UK oninvestment and jobs, adding to a climate of fear, and possiblyswaying some votes to help keep that union in tact.
As a result some voters pledged to boycott firms wading intopolitics, and perhaps wariness from that experience deterredsome of Britain's biggest employers from signing the letter.
Cameron's move to call on the voice of big business - theletter was arranged by his Downing Street office alongside the'Britain Stronger in Europe' campaign - was not without risk as"Out" campaigners cast the EU as the instrument of a globalelite which is out of touch with ordinary Britons.
Jim Mellon, a fund manager who is helping fund the "Out"campaign, told the BBC the signatories were mostly people whohad "crawled their way up the corporate ladder" rather than"true entrepreneurs" who had started new businesses.
The stakes are high in the June referendum. A vote to leavewould not only transform Britain's future in world affairs butalso shake the EU, which has struggled to maintain unity overmigration and financial crises, by ripping away itssecond-largest economy. (Additional reporting by James Davey and Michael Holden;Editing by Guy Faulconbridge and Richard Balmforth)