Staff at the BBC are "morally certain" that Downing Street staff hacked into the corporation's news computers, according to respected foreign correspondent John Simpson.

In a new book being serialised in the Sunday Times, Simpson says that staff at Number 10 were aware of critical news reports before they were broadcast.

But he admits that an investigation into possible security breaches by senior management failed to establish conclusive proof of unauthorised access to the computers.

Downing Street dismissed the allegations as "complete rubbish" and "utter drivel".

Simpson said that on a number of occasions the BBC was contacted by government officials who urged that reports be toned down before they had even been broadcast.

Concern was said to have centred on former corporation employees who may still have known passwords to access the newsroom computers, and shortly afterwards a more secure computer system was installed.

In his book, "News from no man's land", Simpson said that a journalists would write a script on his PC only to be "rung up by Downing Street before it was broadcast and lobbied on a point or two".

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

The BBC said: "We do not discuss issues of security. However we continually review all of our systems and always aim to maintain complete impartiality in our political journalism."

The Conservatives said that if the allegations were correct then Number 10 and Labour Party staff could be complicit in a criminal act.

"At the very least, it would seem that there has been systematic action by Number 10 and the Labour Party to break the BBC's charter which requires independence and impartiality," said shadow deputy prime minister David Davis.

He called for the BBC to publish the results of its internal inquiry and to say what action it had taken.

"From this, we will be able to see whether this inquiry was properly carried out, and what conclusions can be drawn."

Davis also said he would ask civil service chief Sir Andrew Turnbull to determine whether taxpayer funded officials had been involved in "these improper actions".