Mr Anwar was sacked in early September and detained after leading protests against Prime Minister Mahathir Mohamed's 17-year-long rule.

He faces 10 corruption and sexual charges, all of which he has denied. He could receive up to 14 years in prison if convicted.

Mr Blair said in the letter to Wan Azizah Wan Ismail that the world was closely watching his trial and that the UK Government's aim is to ensure that the country's former deputy prime minister is treated fairly.

Mr Blair's spokesman added that the prime minister is appalled by Mr Anwar's treatment in custody.

The letter comes less than two weeks after US Vice President Al Gore angered Mr Mahathir by praising "brave" Malaysians protesting for reform.

Wan Azizah Wan Ismail has vigorously supported her husband's case

Mrs Azizah again hit out at police crackdowns on pro-reform protests by Mr Anwar's supporters.

"You can't blame the supporters of the reform movement. They are not creating riots in the country," Mrs Azizah said.

"We are not trying to create instability. We want freedom of speech. We do not want a police state. The situation becomes chaotic when excessive force is used on the protesters."

Mrs Azizah said the demonstrators came unarmed, and only "carry a raincoat to protect themselves from the water cannon".

She said there were still five people detained under the Internal Security Act, which provides for detention without trial.