It's been a harsh homecoming for Mario Scaramella, the Italian contact of murdered former Russian spy Alexander Litvinenko.

With the blessing of British police investigating Mr Litvinenko's poisoning from radioactive polonium, Prof. Scaramella flew back to Italy on Christmas Eve - only to end up in a Rome jail ever since.

According to court documents obtained by Reuters, Prof. Scaramella is accused by prosecutors of deceiving Italian police about an assassination plot back home and even impersonating a spy.

One witness portrays Prof. Scaramella, 36, using his role as a parliamentary KGB consultant to try to dig up dirt on politicians, including Prime Minister Romano Prodi.

"We're dealing with an absolutely untrustworthy individual," wrote prosecutors in their lengthy justification for Prof. Scaramella's arrest. They added their investigation had shown Prof. Scaramella "constructs false accounts for his own purposes".

Prof. Scaramella denies wrongdoing and his lawyer says his accusations came from reliable sources, like Mr Litvinenko.

A judge last week denied Mr Scaramella bail, citing fears he might flee.

Mr Litvinenko's painful death from radiation poisoning drew a worldwide audience to his fierce criticism of Russian President Vladimir Putin, whom he accused of ordering his assassination.

The Kremlin dismissed the charge as ridiculous. British police are investigating the case as murder, and have visited Moscow to gather evidence, including from two Russians who like Prof. Scaramella met Mr Litvinenko in London on the day he was poisoned.

The investigation in Italy has thrown new light on the relationship between Prof. Scaramella and Mr Litvinenko.

The dead man's brother Maxim, who lives in Italy, told investigators that Prof. Scaramella wanted to use Mr Litvinenko as a source for his research into Italian politicians and allegations of links to the Russian security services.

Mr Litvinenko obliged. According to Maxim's testimony, one of the last things he did for Prof. Scaramella was sit down in front of a video camera last spring in Rome and repeat an accusation that Prof. Prodi may have had ties to Russian intelligence.

Mr Litvinenko sought assurances that the video would not be leaked to the press, and warned that he personally knew nothing about Prof. Prodi, Maxim told investigators.

But then, cameras rolling, Mr Litvinenko said former FSB deputy chief Anatoly Trofimov warned him in 2000 that he should not move to Italy because Prof. Prodi was "one of their men".

"My brother said he didn't have any documents against Prof. Prodi or other Italian authorities," Maxim told investigators.

"He said only that he had spoken with a general from the Russian secret services... (who) responded that in Italy there was Prof. Prodi, then-president of the European Commission, who was one of their men."

Maxim said he was paid €200 cash to translate on the day Prof. Scaramella recorded the video. Prof. Scaramella paid Mr Litvinenko €500-€600 to cover travel expenses.

Prof. Prodi last week said parliamentary commissions like the one Prof. Scaramella worked for should not be created to "slaughter political adversaries". His office has also sent out a statement warning it would sue anyone who slandered him.

The accusations against Prof. Prodi, potentially groundless by Mr Litvinenko's own admission, and impossible to verify since Trofimov was dead, were not included in the parliamentary commission's conclusions before it was disbanded last year. They have also been given no weight in Italy, where Italian newspapers have widely, and sceptically reported on Mr Litvinenko's supposed comments in articles that criticise Prof. Scaramella.

It is still unclear why Mr Litvinenko chose to lend his face to the accusations, although a friend of his suggested that Prof. Scaramella had simply worn him down.

"In the end, (Litvinenko) decided to tell Scaramella what Scaramella wanted to hear," Oleg Gordievsky, a top KGB spy stationed in London who defected to the west in 1985, said in an interview with Italy's La Repubblica.

Prof. Scaramella once wanted to be a spy, but his repeated offers to collaborate with Italy's secret services were all rejected in the 1990s, the government says.

According to the court documents, his first dealing with real-world agents as a parliamentary consultant went badly. In 2003, he paid €5,000- €6,000 for old, useless information on Italian corporate links to Russian intelligence. But the prosecution's case against Prof. Scaramella, the reason he is in jail, centres on Prof. Scaramella's accounts to police of a plot against his life by an ex-Ukrainian agent.

Prosecutors suspect Prof. Scaramella may have made it all up to pressure the ex-agent to give him information, or possibly to make himself seem more credible as a parliamentary consultant.

The ex-agent, Alexander Talik, declined to comment when contacted by Reuters. But in court documents, Mr Talik said Prof. Scaramella once even asked him to sign a letter making false accusations against an unidentified Russian.

The private meeting took place after Prof. Scaramella had told police Mr Talik was trying to kill him.

"He showed me the police statement and then showed me a letter that I should have signed," Mr Talik told police.

"But I didn't do it because they were lies."