The BBC has insisted its coverage is always impartial despite claims that Downing Street hacked into its computer system in efforts to influence journalists.

The Conservatives are calling on the BBC to outline what it did to investigate the allegations.

In a new book, BBC world affairs editor John Simpson suggests the hacking was part of government attempts to influence news reports.

There are a number of ways that people could have been aware of stories that we were working on

BBC spokesman

Downing Street has branded the claims "complete rubbish" and "utter drivel" but Tory frontbencher David Davis says the claims could dent public confidence in BBC coverage.

The BBC says it always acts to stop any abuse of the type suggested, adding that there had been no formal investigation into these claims.

Post-election concerns

Mr Simpson says officials contacted journalists to try to get them to change parts of their scripts before the reports were even broadcast.

The Sunday Telegraph newspaper, which is serialising his new book, News from No Man's Land, says BBC bosses ordered an investigation after staff raised their worries after the 1997 general election campaign.

Downing Street brands the claims "drivel"

The newspaper says the corporation later switched to a more secure computer system.

A BBC spokesman said: "We do not discuss issues of security. If there was any abuse we would put a stop to it.

"There are a number of ways that people could have been aware of stories that we were working on, for example from our talking to potential contributors.

"At all times, our reporting remains impartial."

Sustained pressure?

Mr Simpson says journalists writing scripts for the next news bulletins had been called by Downing Street before their reports were broadcast and "lobbied on a point or two".

"This didn't happen just once or twice," he says. "Downing Street has also rung up the World At One programme to complain about the items it was planning to run."

Several fellow broadcasters were "morally certain" it was part of government efforts to put pressure on the BBC and other organisations, he adds.

The claims prompted David Davis, shadow secretary for the office of the deputy prime minister, to write to Gavyn Davies, chairman of the BBC governors.

Confidence worries

The Tory MP wants to know what has been done to investigate the allegations.

Mr Davis said: "If Mr Simpson is right, there has been attempted interference in the BBC's news coverage using information that should not have been available to either Number 10 or the Labour Party.

"These are serious allegations. The charter requires the BBC to provide independent and impartial news coverage.

"The public needs to be reassured that the BBC has done all it can to ascertain whether any improper conduct has taken place."