BARONESS Thatcher, the former British prime minister, was drawn into the Eurotunnel crisis last night after it emerged that some Japanese banks believe she had promised that no British government would let the project collapse.

Hopes that the British and French governments might eventually rescue Eurotunnel may explain why the 25 Japanese banks have proved resistant to an immediate solution to the company's debt problems.

Senior sources involved in the restructuring talks have spoken of a letter signed by the former prime minister encouraging Japanese investors to support the project.

The news emerged as the governments down played the revelation that Eurotunnel had asked London and Paris to guarantee a bond issue to help free it of its £8 billion of debts. Mr Patrick Ponsolle, Eurotunnel's co chairman, warned the French government that it faced a financial scandal unless it provided the support.

Japanese banks were initially unenthusiastic about investing in the Channel Tunnel when the government was trying to drum up support for the private sector project in the mid 1980s.

A second attempt by the financial advisers to bring them on, board succeeded and, according to one French source, Lady Thatcher's views were central in persuading them. "The letter is vague, but I can see how it might have been interpreted as a promise of government support," he said.

Japanese banks have persistently claimed that they only took part in the project after receiving what they took to be, in effect, a British government assurance that the project would not be allowed to go down.

Ever since Eurotunnel announced it was suspending interest payments on its junior debt, the Japanese bankers in London have been considering whether they might have a claim against the British government.