Sir Willie Morris, the British ambassador to the Kingdom of Saudi Arabia, was not a man to mince words in private. On February 21, 1970, he sat down in his embassy in Jeddah and dictated a secret telegram to a senior official in the defence ministry in London.

Some of what he said was hair-raising: he predicted among other things that the “present regime”, a staunch ally of the West, was in increasing danger of being overthrown although it was “probably good for another five years”. But his main purpose was to discuss British arm sales to the Saudis and corruption.

Morris pointed out that, as far as Britain’s reputation was concerned, “our name stinks”. Just why is not known because the relevant pages were retained by Whitehall when his memo was placed in the national archives.

Enough was released, however, to reveal that he regarded arms sales in Saudi Arabia as a “crooked world”. He added: “It is a jungle inhabited by beasts of prey in which one must move with caution and uncertainty.”

The ambassador said he had considered reporting the corruption to King Feisal, who was then on the throne but was soon to be assassinated, and asking him to intervene. But “we would be unlikely to succeed, and . . . in failing we would antago-nise both him and his brothers”.

He concluded: “Commercially and politically, the ‘system’ is at best an infernal nuisance, and it is potentially explosive — a timebomb under the regime . . . If there were a practical way of extricating ourselves, we should take it.”

Britain did not heed his advice. Fifteen years later, in 1985, Margaret Thatcher sealed a £40 billion arms deal with the Saudis, who by then were immensely rich after the massive oil price rises of the 1970s. They called the deal Al-Yamamah or “the dove”.

It is one of the delightful ironies of time that the head of the diplomatic service during that arms deal was Sir Antony Acland, who as a young Arabic department high-flyer had scribbled on Morris’s memo: “This is an admirably clear and comprehensive survey of the complications.”

Another 20 years on, Morris is dead, Acland is in illustrious retirement, having been both ambassador to Washington and provost of Eton, and the “timebomb” is set to explode as British fraud investigators probe Al-Yamamah.

Among those embarrassed is Wafic Said, the Syrian-born businessman who is among Britain’s most generous philanthropists. He has acknowledged being a “middleman” in the 1985 deal but has always denied making any money from it.

Saudi Arabia, however, is reported to have warned that unless the investigation into possible corruption in Al-Yamamah is called off there will be dire repercussions.

Specifically, it will abandon a new multi-billion-pound weapons agreement to buy British-made Eurofighters — for Britain still ventures into the Saudi “jungle” — and it may even go so far as to break diplomatic relations.

This is causing a moral dilemma for Tony Blair’s government. Which is most important to the public interest: to uphold the rule of law or to bow to a crucially important Middle Eastern ally and secure another £10 billion arms deal?