Immigration officers are questioning Tube travellers because they sound "foreign", the Evening Standard has learned.

Thousands of passengers are being stopped in a secret operation using tactics the police are specifically forbidden from deploying. Immigration officers are stopping anyone they consider to look or sound foreign and asking them to produce their papers to prove their right to British residence.

Their aim is uncover illegal immigrants and failed asylum seekers. The discovery that these tactics are being used prompted a political row today. The Liberal Democrats are set to write to the Home Secretary demanding an explanation for a scheme civilrights groups dubbed "Stalinist".

The existence of the spot-check operation has been kept secret by the

Home Office but an Evening Standard investigation discovered that teams of immigration officers have been carrying out the procedures since May 2003.

It is part of a wider programme in which 1,000 suspected illegal immigrants have been detained.

During one operation witnessed by the Evening Standard, a series of people getting off Tube trains were stopped by immigration officers dressed in body armour and carrying handcuffs.

The officer in charge said people were picked out for questioning if they sounded foreign.

One immigration officer said: "If you hear someone speaking a language that's not European we approach them and ask 'do you mind if I ask you what nationality you are?'

"If they get upset or start acting suspiciously we ask the police to assist and demand identification."

Onlookers said they were shocked when they saw the operation at Harrow-on-the-Hill station shortly before the evening rush hour.

Ellen Cook, 23, an admin assistant, said: "They are assuming if people look different they should be harassed. It's despicable."

Today the Home Office faced a barrage of criticism over the tactics. Police are specifically forbidden by Home Office guidelines from stopping people because of their accent or appearance.

A junior Home Office minister promised an investigation when official figures showed a massive jump in the number of Muslims stopped by police under anti-terrorist laws.

Mr Blunkett now faces demands for an explanation of his secret initiative.

Mark Oaten, home affairs spokesman for the Liberal Democrats, told the Evening Standard he was writing to Mr Blunkett to demand answers. "This is a step too far," he said. "The Home Secretary should urgently review powers that allow immigration officers to indiscriminately stop people in this way."

Barry Hugill, of the civil rights group Liberty, said: "Stopping people and checking their papers is what we used to associate with the old communist states.

"It's real stab-in-the-dark stuff. It is not intelligence-led, it's 'lets see what we can get today'."

Keith Best, chief executive of the Immigration Advisory Service, said: "It is a time-consuming and costly way of ending up with just a few people arrested by circumstance.

"It's a rather random blunderbuss approach that offends many ordinary decent citizens, some of whom will be third or fourth generation British."

The Home Office said it had no figures for how many illegal immigrants had been caught in the sweeps of Tube stations.

British Transport police officers are taken away from regular patrol duties to back up the immigration officers.

Today the Home Office defended the policy.

A spokesman said: "The Government has made it clear it will take a robust stance against those who abuse UK laws.

"We will prosecute and will seek to remove those who have no legal basis of stay in the UK."