Mr Letwin warned of the dangers of unofficial censorship

Oliver Letwin was responding to the revelation on Sunday that John Gieve, permanent secretary at the Home Office, wrote to BBC chairman Gavyn Davies to complain about The Secret Policeman a month before it was broadcast.

The Observer newspaper said unnamed senior BBC executives viewed the move as an attempt to get the programme pulled.

A flurry of police resignations and suspensions followed the airing of the film, in which one policeman wore a mock Ku Klux Klan hood and insulted the parents of murdered black teenager Stephen Lawrence.

The functioning of a democracy cannot be taken for granted if the state seeks to exercise unofficial censorship Oliver Letwin Shadow home secretary

The Home Secretary David Blunkett initially attacked the methods used in the programme, calling it a "covert stunt", but later conceded the behaviour captured on film was "horrendous".

'Intervention'

The documentary was based on a seven-month undercover operation by BBC journalist Mark Daly, who joined Greater Manchester Police as a trainee officer.

The reporter was arrested on suspicion of obtaining a pecuniary advantage by deception and damaging police property. He is due to answer bail next month.

We look to the police for protection, advice and security - it's for this reason I believe our investigation was justified Mark Daly My life as a secret policeman

On Sunday, Mr Letwin praised the BBC's "valuable role in uncovering sickening and intolerable examples of racism in that particular police force.

"It is therefore extremely disturbing to discover that attempts were made by the Home Office, at the highest level, to intervene in relation to this film some weeks before it was shown," he said.

Mr Letwin said he would table questions in parliament to find out the intent of the intervention.

"If the Home Office was not trying to keep the film off the air, what was it trying to do?" he said.

"We need a clear answer because the functioning of a liberal, tolerant and law-abiding democracy cannot be taken for granted if the state seeks to exercise unofficial censorship over important journalistic investigations."

'Diversion'

The Home Office confirmed Mr Gieve sent the letter on 12 September, but denied it was an attempt to force BBC bosses to axe the programme.

"John Gieve did not ask the BBC to pull the programme," a spokeswoman told BBC News Online.

"He was expressing concerns about the undercover methods used in it."

She accepted the programme had "raised serious issues, which we are acting swiftly to tackle".

A BBC spokesman said the corporation "does from time to time come under pressure from institutions or government bodies about the content of its investigative programmes.

"The BBC will not pull back from making demanding and challenging programmes."

He said this latest row only served to divert attention from the issues brought up in the film.

Crimewatch 'threat'

Sunday's Observer article also alleges that the chief constable of Greater Manchester Police told the BBC it risked a new "Hutton-style inquiry" if it went ahead with the broadcast.

Chief constable Mike Todd is said to have met with the BBC director-general Greg Dyke and other senior executives a week before the film went out.

He is alleged to have warned the BBC it risked destroying its relationship with the police.

He is also said to have threatened to withdraw police cooperation in the BBC1's Crimewatch programme.