The men were all convicted of supplying class A drugs after they unwittingly sold them to officers posing as addicts in Peterborough, and were locked up by a judge at the city's crown court.

They were among 11 people who pleaded guilty to drugs offences yesterday as a result of Operation Limestone, which culminated in a dramatic series of co-ordinated police raids on the homes of suspected dealers on a single day in November.

The officer in charge of the investigation, Detective Sergeant David Liddle, said the sentences should act as a warning to anyone thinking of selling drugs in the city.

He said: "We are satisfied with what has happened in court, and we are looking forward to seeing the rest of those who have pleaded guilty given similar sentences.

"We want to send out a message to others to let them know they can expect to be punished with similar severity if they get involved in drug dealing in Peterborough."

The court was told that two undercover officers were sent into the city in September and October to pose as heroin addicts and infiltrate its drug-dealing community.

The detectives, known as Kayleigh and Lee, made contact with several dealers, some of whom approached them in the street in the city centre, Millfield and Eastgate areas touting for business.

After earning the trust of the dealers, they bought small quantities of heroin or crack cocaine from them while secretly videoing and taping their encounters.

The operation ended abruptly at dawn on Thursday, November 6, when police conducted a series of raids on homes across the city.

Faced with the overwhelming weight of evidence against them, all 11 of the suspects who appeared in court yesterday pleaded guilty to drug dealing charges.

Michael Varone (47), of St Paul's Road, Peterborough, who is known on the street as Mick, was sentenced to six years in prison after he pleaded guilty to six counts of supplying heroin.

Keith Thomas (20), of Balance Road, in Hackney, London, regularly visited Peterborough to sell drugs and was described as an "extremely active and skilful dealer", sending text messages to his customers advertising "lovely gear", "guaranteed good sizes" and "dat 9/10 stuff".

Thomas, known as Marcus, was given a four-and-a-half year sentence after admitting eight charges of supplying heroin and two of offering to supply class A drugs.

Masood Ali (30), known as Ali, of Field Walk, Peterborough, and Ameed Hussain (20), known as T, of Windmill Street, Millfield, Peterborough, were jailed for three years and two-and-a-half years respectively.

Another man was given a community sentence and six others will be sentenced at a later date, with several more suspects set to face similar charges in the coming weeks.