IN a bid to curb teenage crime, Britain is about to introduce ''boot camps,'' American-style. Home Secretary Michael Howard plans to open the country's first ''punishment center'' next year at Warrington, Cheshire. Youths will face a 16-hour day of work, physical exercise, and school lessons. Yet unlike boot camps in the United States, where a rigorous drill-sergeant approach is used, their regime would ''stress rehabilitation rather than punishment,'' says one of Mr. Howard's officials. But Howard - known for his tough talk on law-and-order - says he wants the worst offenders to get compulsory military-style punishment in an actual Army prison in Colchester, Essex. He has also talked with the Ministry of Defense about accepting hardened young criminals for what a Home Office official said would be ''severe punishment, Army-style.'' The government of Prime Minister John Major, aware of rising public concern about teenage crime and of upcoming general elections, will be looking for fast results. In London, according to Police Commissioner Paul Condon, one-third of all crimes are committed by people under 20, and 13 percent by those who are in the 14-to-16 age group. The latter took part in 53 percent of street robberies and 50 percent of vehicle crimes last year. Predictably, the modified boot camps have been dismissed as ''pointless'' by Jack Straw, a member of the opposition Labour Party who monitors the Home Office. But not so predictably, Mr. Straw says that three of Howard's officials toured five American boot camps in May 1994 and reported back that the system in force there was degrading. Straw claims to have seen a copy of their findings, which, he says, argues strongly that American ''houses of pain,'' with their constant tongue-lashings and calculated humiliation of inmates, are counterproductive and unsuited to conditions in Britain. A Prison Service spokeswoman agreed that the officials' report exists, but said it was ''not customary to pass confidential advice to Parliament.'' It seems clear that in opting for a relatively gentle regime in the first British boot camp, Howard reluctantly accepted the findings of his own officials' report. Straw and other critics of the government's prison policy now claim that he wants bad offenders to receive Army-style punishment at Colchester because he has to demonstrate to Conservative Party supporters that he has not gone soft on youthful crime. If he persuades the Ministry of Defense to take civilian prisoners, the young inmates can expect a tough time. Detainees at the Military Corrective Training Centre at Colchester spend their days scrubbing floors, polishing windows and furniture, and folding blankets. Strict discipline is enforced by a staff with powers to mete out extra punishments. Some juvenile-crime experts warn against Howard's approach. ''The idea may be attractive to the Conservative Party faithful, but in reality it will be a disaster,'' says Harry Fletcher, assistant general secretary of the National Association of Probation Officers. John Major's government will try to crack down on young offenders without going too far.