Jerome Drean, 34, head of European equity derivatives trading at Credit Suisse and who earned four million US dollars in three years, travelled from his home in London to impersonate Elnar Askerov, 23, an economics student who was struggling with his studies.

Both men were sentenced to nine months imprisonment suspended for two years at York Crown Court yesterday where Judge Stephen Ashurst was told Drean was caught when an exam invigilator realised he was not the student he had taught.

They both admitted conspiracy to defraud York University. Drean, who had already repaid £4,000, was ordered to forfeit £16,000 as the proceeds of crime.

Each was ordered to do 300 hours unpaid work and pay costs of £1,360.

In January 2006 the pair struck up an agreement whereby Drean would travel from his London home by train to York to sit Askerov's economics exams.

The scam succeeded on eight occasions until one of Askerov's final exams last May, when a lecturer spotted Drean.

Robert Smith QC, for Drean, told the court his client was "vastly intelligent" but suffered from Asperger's Syndrome which could lead to socially unacceptable behaviour.

Mr Smith added: "He did not want to do what he did, but having been given an invitation he seems to have seen it as some sort of challenge."

Drean, of Islington, London, would never get another job in a British financial institution, where he had been regarded as a leading figure, and his life was now in ruins.

He told Judge Ashurst: "To send him to prison as well would add nothing to the burden which is self imposed by him."

Alexander Cameron QC, for Askerov, of Acton, London, said his client had felt a pressure to achieve: "He will return to Azerbaijan without any qualifications having let down his family."

Judge Ashurst said: "The court simply does not know how serious a problem examination fraud of this kind may be. If it's widespread of course it erodes the confidence the public can have in academic degrees.

"Your conduct in particular must be deeply offensive to the thousands of diligent students who do work hard."

The judge added: "The idea that degrees can be bought and sold must be discouraged."

A spokesman for York University said: "This was a sophisticated deception by a determined and unscrupulous individual. Such breaches of trust, though rare, are nevertheless a betrayal of the overwhelming majority of our students.

"The university is conducting a detailed review of its procedures relating to the administration of examinations."