The explicit accusation is contained in a draft report to a parliamentary commission on terrorism.

The formerly communist LDP is the biggest party in Giuliano Amato's centre-left government, and the report could sour relations between Italy and the United States and unleash a storm of domestic political controversy.

The 300-page report says that the United States was responsible for inspiring a "strategy of tension" in which indiscriminate bombing of the public and the threat of a rightwing coup were used to stabilise centre-right political control of the country.

Those who carried out the attacks were rarely caught, it said, because "those massacres, those bombs, those military actions had been organised or promoted or supported by men inside Italian state institutions and, as has been discovered more recently, by men linked to the structures of United States intelligence".

Valter Bielli, a Left Democrat member of parliament and one of the authors of the report, said his party's conclusions were based on recent judicial discoveries and a re-elaboration of information that had been available for many years but had not been adequately understood.

"I am convinced that the intervention of the Americans in Italy is now a historically proven fact," he said.

"They interfered to prevent the Communist party from achieving power by democratic means. The communist threat no longer exists and it would be appropriate if the Americans themselves helped us to clarify what happened in the past."

Mr Bielli said he was worried about the possible implications of the report for relations between Italy and the US, but he hoped it would contribute to the creation of a new Nato in which all countries enjoyed equal weight and dignity.

"During the cold war the east was under communist domination, but the west too had become, in a certain sense, an American colony," he said.

The report claims that US intelligence agents were informed in advance about several rightwing terrorist bombings, including the December 1969 Piazza Fontana bombing in Milan and the Piazza della Loggia bombing in Brescia five years later, but did nothing to alert the Italian authorities or to prevent the attacks from taking place.

It also alleges that Pino Rauti, a journalist and founder of the far-right Ordine Nuovo (new order) subversive organ isation, received regular funding from a press officer at the US embassy in Rome.

"So even before the 'stabilising' plans that Atlantic circles had prepared for Italy became operational through the bombings, one of the leading members of the subversive right was literally in the pay of the American embassy in Rome," the report says.

Mr Rautinow heads the small rightwing MSI Fiamma-Tricolore party, and suggestions that he and other rightwing politicians still actively involved in parliamentary politics had failed to cut their links to terrorist extremists have drawn furious rebuttals from the centre-right opposition.

The National Alliance leader, Gianfranco Fini, described the document as a "miserable report" and the centrist Republican party said it was worthy of a 1970s Maoist group.

"These are allegations that have come up over the last 20 years and there is absolutely nothing to them," a source at the US embassy in Rome said.

To Aldo Giannuli, a historian who works as a consultant to the parliamentary terrorism commission, the release of the Left Democrats' report is a manoeuvre dictated primarily by domestic political considerations.

"Since they have been in power the Left Democrats have given us very little help in gaining access to security service archives," he said.

"This is a falsely courageous report. The real issue today is gaining access to Nato's archives. There has been no impulse on this front from the government."