Britain's Labour Party takes sharp swing to the right by dumping leading left-wingers

In what amounts to an internal constitutional coup, the commanding heights of the British Labour Party are suddenly in moderate hands.

A series of votes in the party's powerful National Executive Committee (NEC) has pushed leading left-wingers out of key positions and replaced them with members of the center-right.

Tony Benn, only a year ago a seemingly dominant left-wing chieftain, is the most prominent casualty. He has been voted out of a committee that works with the trade unions to determine Labour policy on economic and industrial issue.

Mr. Benn is also expected to lose the chairmanship of the party's Home Policy Committee.

Another influential left-winger, Eric Heffer, is virtually certain to be replaced as chairman of Labour's Organization Committee, which will determine the fate of Militant Tendency, a hard-left ''party within the party'' largely run by Trotskyites.

The sharp swing to the right in the NEC has its roots in this year's Labour Party conference, which placed control of the party in moderate hands. Since the conference, leading moderates have been preparing a thoroughgoing purge of left-wingers in key positions.

When the time came to decide membership of subcommittees, the party leader, Michael Foot, found himself unable to curb the enthusiasm of the right wing.

In the NEC vote Mr. Foot abstained throughout. His advisers say he is unhappy about the outcome, which seems certain to cause widespread bitterness inside the Labour movement.

The right-wingers, however, say the left has no justification for being upset. When they held sway in the NEC, they were just as ruthless.

Mr. Benn took his defeats calmly. He has announced that he will stand for election to Labour's shadow cabinet, but Westminster insiders say he may be lucky to gain a place.