Christer Pettersson said he shot Olof Palme

On Sunday the late prime ministers' widow, Lisbet Palme, and his son, Maarten, said they would press for the case to be reopened.

Mrs Palme was the main witness in Petterson's 1989 trial, and picked him out in an identity parade.

On Thursday she said in a newspaper interview that she knew who the murderer was.

However, Swedish legal experts have questioned the credibility of Petterson's confession of a 54-year-old former convict with a history of alcohol and drug abuse.

Sure as hell it was me who shot [Palme], but they can never nail me for it. The weapon is gone

Christer Pettersson

He has made similar statements in the past, though always stopping short of a full confession.

This time he said: "Sure as hell it was me who shot [Palme], but they can never nail me for it. The weapon is gone."

Olof Palme was shot dead in central Stockholm on 28 February 1986, when he and his wife Lisbet were walking home from the cinema.

The killing which stunned Sweden has never been solved and the murder weapon never found.

A former prosecutor in the case, Anders Helin, described Mr Pettersson's confession was a "PR-trick that does not mean anything".

Experts also say that Mr Pettersson has been paid large sums of money for his comments to the media.

A criminal law professor, Christian Diesen, said "there was no substance in the confession in its current form."

"He [Pettersson] wants to be a person of importance -- perhaps a prime minister murderer," he added.

In legal limbo

Under Swedish law, the case can be reopened if significant new evidence is produced.

Such evidence may be a full and credible confession of the crime.

But Mr Pettersson can not be forced to repeat his confession to the police because he was acquitted of the murder.

Palme investigators said they wanted to question Mr Pettersson, but he would have to come voluntarily.

They stressed that, even then, a confession alone would not be enough to reopen the case.

The prosecutor in charge of the Palme case, Agneta Blidberg, said that "he must say something about the weapon because the appeals court set that condition in its ruling."

"That is the only technical evidence that could be cited as a reason to reopen the case," she added.