British secret service under fire. Charges MI5 spied on Britons for political purposes are said to be causing major shakeup

Britain's trouble-prone secret service is attracting renewed criticism following highly publicized charges that the counterespionage organization, MI5, has been using its intelligence apparatus for political purposes. The MI5, according to a documentary scheduled for broadcast on the British ``20/20 Vision'' television program, has been tapping the telephones of trade union leaders, antinuclear campaigners, and radical journalists. The program was banned at first by the Independent Broadcasting Authority, which feared that it contravened the Official Secrets Act.

The documentary relies heavily on comments by Cathy Massiter, an operative with MI5 for 14 years before she left the service in 1984.

Ms. Massiter claims the MI5 assigned her to investigate the Campaign for Nuclear Disarmament (CND), and that her findings were later used by leading government politicians to defend the Thatcher government's nuclear policies.

Massiter also claims that CND was infiltrated by MI5 in contravention of MI5's own guidelines, and that telephone wiretaps had been ordered on many leading CND and trade union figures.

The MI5 also opened files on British citizens, including journalists, thought to have radical leanings. The implication of Massiter's allegations was that MI5's role as a counterespionage body had been broadened to include the improper surveillance of average citizens.

When the TV program was banned, she repeated her charges to British newspapers. The allegations were taken up by the opposition Labour Party, which forced Prime Minister Margaret Thatcher to order an urgent inquiry headed by Lord Bridge, a man with substantial intelligence experience.

Lord Bridge, who quickly reported that no illegal phone taps had been made by MI5, cleared the way for the television program to be screened. By that time, moves were reported to be afoot to order important behind-the-scenes changes in the running of MI5.

A succession of intelligence leaks and spy scandals over the past few years have raised serious doubts about the efficiency of the nation's undercover agencies. These doubts were compounded by Mrs. Thatcher's agreement to station US cruise missiles on British soil -- a move that helped to turn CND into a mass protest organization. The alleged abuses arose in the political climate generated by the escalating nuclear protest movement.

Massiter says she became disillusioned by MI5's tendency to stray into domestic surveillance. Official secrecy and reticence make it hard to determine the extent of the problem, but there are reports that Mrs. Thatcher has ordered a shakeup of MI5.

Among the actions she is said to back are:

A requirement that a rigid distinction be made between authorized telephone ``tapping'' and more or less free-lance ``bugging'' of individuals' phones.

A sharp reminder to MI5 and politicians alike that the intelligence services must not be used for overt political purposes.

Urgent steps to give the intelligence services a ``spring cleaning'' at command level, replacing some long-serving officers.

The Thatcher government says there will always be a need in Britain for legitimate intelligence activities, including close surveillance of known communists in influential positions. But the furor whipped up by Massiter's allegations has warned the prime minister that there are limits beyond which British citizens do not want MI5 and other covert agencies to step. -- 30 -- {et