The Media Column: The empty chair plays a big part behind the scenes at <i>Today</i>

When John Humphrys interrogates his guests on Mastermind, they are obliged to face his questioning while sinking into a black leather armchair made famous by Magnus Magnusson in the Seventies. Comfy though it may look, that chair - introduced with the ominous theme tune - has come to represent the most daunting mental challenge that British popular culture has to offer.

In a country that has resisted the inclination to introduce the equivalent of Florida's 2,000-volt Old Sparky, the Mastermind chair is reckoned to be one of the scariest pieces of furniture in Britain. But Humphrys, who revels in his role as the BBC's inquisitor general, has an even more frightening seat stored away in his warehouse. He brings it out for the Radio 4 Today programme, and it is usually reserved for government ministers. It is the dreaded "empty chair".

Even though it exists only metaphorically, the empty chair has become increasingly prominent in the behind-the-scenes negotiations at Today and is a symbol of the programme's more aggressive approach to its journalism. Two years ago, in his column in The Sunday Times, Humphrys explained the threat of the "empty chair". "We use it as a verb," he said, noting that, although the concept had its origins in the Fifties, when a tobacco-industry representative failed to turn up for a debate and was replaced with a piece of furniture, its use by Today was "a relatively recent phenomenon". Humphrys said: "We've become much more robust in the past few years about telling the audience when a politician ducks an interview that the editor thinks they have a right to hear."

So, ministers are increasingly faced with the option of getting up at five or six o'clock in the morning to take part in a radio debate (often helping to raise the importance of minor stories) or having listeners told that they have gone into hiding.

Press officers say Today takes a similarly aggressive approach to embargoes, demanding that they be set to allow the programme to broadcast the news "exclusively", before any other media outlet.

Such a hard-ball policy has helped to make Today the agenda-setting news programme that it undoubtedly is. It has also helped to create an environment of arrogance, to the point of recklessness, in which the BBC's shortcomings exposed in the Gilligan affair were accidents waiting to happen. As a result, it has been the BBC's director of news, Richard Sambrook, who has found himself squirming publicly in the hot seat.

IT HAS BEEN a difficult time for BBC news all round. At the weekend, it emerged that one of its journalists had been arrested after taking a job as a police constable. The undercover exercise was meant to expose institutional racism in Greater Manchester police but backfired spectacularly.

Going into disguise to infiltrate a gang or an organisation has long been the glamour end of investigative journalism, and advances in technology have made that approach increasingly attractive to television, where the results of secret filming can best be displayed. But such reporting is potentially dangerous to the journalist's reputation, as well as their physical well-being. The grubby end of Fleet Street abounds with legendary tales of reporters being told to black up, cross-dress or forsake their usual bathing routines in order to infiltrate groups deemed to be subversive or likely to generate good copy. One hack went native on his paper and adopted a traveller lifestyle after being told to go on the road with the bus convoys.

If the undercover operation goes to plan, the infiltrated party is likely to know nothing about it until the day before publication or broadcast, preferably as arrests are made or a government inquiry is announced.

The News of the World, whose star reporter, Mazher "The Fake Sheikh" Mahmood, dresses up with an enthusiasm not seen since the days of the cartoon character Mr Benn, recently achieved such an outcome when it infiltrated the Prison Service. The reporter David McGee was able to work unchallenged at Woodhill high-security prison and stood guard over the Soham murder suspect Ian Huntley. When the tale came to light, David Blunkett, the Home Secretary, ordered an investigation and the paper had a sensational story.

The BBC was no doubt hoping for similarly spectacular revelations when it sent a journalist, Mark Daly, to join the police and secretly film his new colleagues. The corporation had already had one narrow escape, after the reporter Claudia Murg was disguised as an asylum-seeker for an exposé of the immigration system. Murg found herself nabbed by the immigration authorities, but only - fortunately for the BBC - after she had gathered sufficient footage of the black-market economy for a Panorama "investigation".

After Daly was held, Chief Constable Michael Todd moaned that cases in which the journalist had been involved might have to be dropped. Presumably, the BBC's story will also be dropped. Unless, of course, there were other undercover reporters of different ethnicity stowed away in Greater Manchester police and Daly was the only one to be picked on.