Banking giant Barclays and the boss of Royal Bank of Scotland have said they want the UK to remain in the EU.

But other big companies including Sainsbury's, Tesco and British American Tobacco have decided to stay neutral.

They were all among big names not included on a letter from big businesses and business leaders supporting the UK's continued membership of the EU.

In a statement, Barclays said: "On balance we think it is in the interests of our customers and clients for the UK to remain in the EU. We have modest interests domestically on the continent, but provide significant services to European companies from London.

"More importantly, we are heavily reliant on a successful UK domestic and international economy and feel this is enhanced through the UK's membership."

A bank spokesman said policy meant it did not sign "letters of that nature".

Offering a personal view, Royal Bank of Scotland chief executive Ross McEwan said: "I can't see an economic scenario where coming out of the EU is a good thing for the bank or the banking industry as a whole in the short to medium term."

The bank did not sign the letter because it has yet to agree a corporate view, a spokesman said.

Some members of the Prime Minister's business advisory group, which includes senior business executives who provide "high-level advice" to David Cameron, were also missing from the list of signatories, although most of the 20-strong group backed the letter.

Advisory group member Robert Noel, chief executive of Land Securities - whose property portfolio includes hotels and shopping centres - said that although he had not signed the letter, he "still supports David Cameron and the EU".

A spokeswoman said Mr Noel did not sign because the letter was "very general" and it was "only in a position to speak about the impact on property markets in a domestic setting".

The spokeswoman added: "If Britain voted to leave the EU, it would have an impact on London markets... companies wouldn't know what was happening and they would move offices. It would have a negative impact."

Other advisers who did not sign included Jeff Fairburn, who runs housebuilder Persimmon, and Liv Garfield, the Severn Trent boss.

Jonathan Smith, external communications manager for Severn Trent, said Ms Garfield's decision was hers, adding: "It is a matter for the individual voter and not something we would comment on. We don't feel we should get involved, but our customers should be involved."

A spokesman for Persimmon Homes said: "We have thousands of employees and customers and no doubt there are strong opinions on both sides. The board has discussed it, and Persimmon has decided to remain firmly neutral."

The company of another adviser, Nigel Wilson, chief executive of Legal & General, is "undecided so far", a spokesman said, adding: "We have always said we would discuss the matter after the PM returned from Brussels with a deal.

"The board will therefore discuss and decide our position in due time."

Also undecided is Costa Coffee and Premier Inn owner Whitbread, whose chief executive Alison Brittain is on the advisory board. "We have a board meeting this week to discuss our position," a spokeswoman said.

Although Asda chief executive Andy Clarke has signed the letter its big supermarket rivals have stayed neutral.

A Sainsbury's spokesman said: "We are an apolitical organisation and feel that the EU referendum is a matter for the British people."

Tesco said: "The referendum on EU membership is a decision for the people of Britain. Whatever that decision is, our focus will continue to be on serving customers."

And supermarket Morrisons said: "Morrisons is neutral on the issue of Britain's EU membership as we believe it is a matter for the people of Britain to decide."

Among FTSE 100 firms which did not sign, British American Tobacco said: "From our point of view it will be interesting to see what happens, but ultimately we are in business and not politics."

And global commodities giant Glencore said: "This is a decision for the British public to decide."

Aberdeen Asset Management, a FTSE 100 investment firm, said a vote for a "Brexit" on June 23 will have no impact on its business in Europe.

Valued at £4.6 billion, the company said that the outcome of the referendum would prove irrelevant to its business operations with Europe, because it has spent years dealing with "differing tax, legal and regulatory systems".

Other big UK employers Specsavers and John Lewis did not want to comment.