LONDON — The British government proposed on Monday stopping the early release of hundreds of terrorism convicts after a succession of attacks by people let out halfway through their prison terms underscored long-festering problems with its anti-terrorist strategy.

Under the government’s extraordinary plan, about 220 people currently locked up on terrorist offenses in Britain would have their prison terms extended, said Robert Buckland, the justice secretary.

Instead of automatically being released halfway into their sentences, as is customary for many offenders in Britain, terrorism convicts would be forced to serve at least two-thirds of their terms, he said. And even then, they would only be released with the agreement of a parole board.

But it was not clear the government’s proposal would withstand scrutiny in the courts, let alone solve a crisis that analysts said had more to do with a decade of austerity cuts to prison and probation services than with the length of sentences.