Avram Grant was sent a package containing a mysterious powder

A note addressed to Grant was opened by a member of staff. It was found to contain anti-Jewish insults as well as claims that the powder was lethal.

A police source said it used the words: "When you open this letter you will die a very slow and painful death."

The powder was analysed by specialists and found to be harmless, police said.

The letter also included threats to the manager's wife Tzofit, a well-known personality in Israel.

A spokesman for Surrey Police said: "We were called to Chelsea's training ground at Cobham at 12.15pm yesterday after they received a suspicious package."

The club's training ground at Cobham, Surrey was sealed off while investigations were carried out.

Detectives are continuing to try to identify the source of the package.

In a statement, Chelsea said: "The required procedure for dealing with such matters was immediately activated and the police and other services were called to handle the situation.

"A harmless substance was discovered. The matter is still being investigated."