Police have launched an investigation after Liverpool manager Gerard Houllier received a death threat in the post last month, the English premier league club said on Wednesday.

Houllier received the threat in a letter he opened at the club's Melwood training centre three weeks ago and Liverpool reported it to Merseyside police, the club said.

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"It was worrying enough to involve the police," a Liverpool spokesman told the club's website.

"They are still investigating. It was very unpleasant and because of that we have involved the police in the situation. We do not want to over-dramatise the incident, but such letters cannot be ignored."

Merseyside Police confirmed "an investigation was launched after a threatening letter was sent to Gerard Houllier...at the beginning of February and the investigation is ongoing."

According to newspaper reports, the letter was anonymous and said the Frenchman should resign as Liverpool manager or his life would be in danger.

Houllier is in Sofia with his team for their UEFA Cup third round second-leg match against Levski on Wednesday night.

Liverpool are 2-0 up from the first leg but the UEFA Cup is their only chance left of a title this season. They are out of the FA Cup and the premier league title race and pressure has been heaped on Houllier to quit.

After Liverpool lost at Portsmouth in an FA Cup fifth-round replay last month graffiti scrawled on the wall at the club's training ground included the message: "Hope you die of Aids, Houllier".

Houllier, 56, has been in charge at Liverpool since 1998. His side won the treble of FA Cup, League Cup and UEFA Cup in 2001 and they won the League Cup again last year but Liverpool have not won the league championship since 1990.