Suspected illegal immigrants arrested by police are being told to find their own way to screening centres - with the help of a map.

Immigration staff, too busy to respond to calls for help, are telling forces who seize suspects to "direct them to the nearest asylum screening unit".

At least one force, in North Wales, was faxed a map pinpointing the location of the unit in Liverpool and instructed to give it to the illegal entrants.

The astonishing guidance was branded a "farce" by the Conservatives, who revealed it at the Commons yesterday.

Cheryl Gillian, a shadow minister, said: "Police are arresting illegal immigrants and being told by Home Office officials to let them go with instructions, and a helpful map, of how to get to the immigration offices in Liverpool.

"They have been quoting paragraph 23.1 of the immigration and nationality directorate's operations enforcement manual.

"Isn't this just another farce being played out by the Home Office, but one that this time is threatening the safety and security of people?"

The guidance is given when suspected illegal immigrants tell arresting police officers that they wish to claim asylum.

Paragraph 23.1 states: "An enforcement office will not always be able to assist the police when suspected illegal entrants come to notice who indicate to the police they wish to claim asylum.

"Where an enforcement office is unable to offer assistance to the police in interviewing, for example, a group of clandestine illegal entrants, and it is not otherwise intended to bail the individuals or deal with them via faxed notices, the police should be advised to direct them to the nearest asylum screening unit."

Richard Brunstrom, the North Wales chief constable, is believed to be furious at the guidance and has written a personal letter of protest to Home Secretary John Reid.

However, Cleveland Police said it had not received the same advice and insisted no suspected illegal immigrants had been let go.

And a Home Office spokeswoman insisted it was only on "the rare occasion"

that staff from the immigration and nationality directorate (IND) were unable to respond to requests.

She added: "Such occasions might include circumstances where immigration staff are engaged in an operation to target illegal work, or removing failed asylum seekers."

"Clearly, if we are unable to pick them up, then yes, the police will let them go. On the rare occasions where that has happened, then that is allowed."

The spokeswoman said there was no specific Home Office guidance that maps should be handed out to asylum-seekers.

The row blew up as immigration minister Liam Byrne told MPs and peers that clearing the backlog of asylum cases could take up to five years.