London — Britain's opposition Labour Party is plunging into the general election campaign with a visibly divided leadership. Party leader Michael Foot, speaking soon after Prime Minister Margaret Thatcher decided on a June 9 election, vigorously denied that the Labour movement was dominated by left-wing figures bent on changing the face of British society.
But one of his most influential opponents, Tony Benn, known as ''the darling of the left,'' countered by declaring that the coming general election was the most important for Britain this century and required Labour to launch a ''moral crusade'' against the Thatcher government.
Despite urgent attempts in recent weeks to paper over differences between the right and left wings of the party, there are early indications that Labour is entering the campaign split on a variety of crucial issues. These include British membership of the European Community and British retention of its nuclear weapons.
Officially, the Labour Party is committed to quitting the European Community and getting rid of the British independent nuclear deterrent. But powerful figures in the party's front rank, including deputy leader Denis Healey, want to soft-pedal these issues and concentrate on what they say is the Thatcher government's failure to curb high levels of unemployment.
Left-wing figures like Mr. Benn, however, will insist that the party campaign strictly on its manifesto - the document outlining the policies it will implement if elected.
For party leader Foot, the rifts already appearing in Labour's ranks are a grave embarrassment. Mrs. Thatcher will lead a fully united Conservative Party and will be able to point to serious inconsistencies in her main opponent's policy attitudes.
The Labour leader has already swerved away from earlier statements that he would fight a restrained campaign. Instead, he has accused the Conservative Party deputy leader of resorting to tactics in which scares and lies were used.
Mr. Foot used this language after the Tory deputy leader, William Whitelaw, had described Labour policies as ''mad'' and ''dominated by the extreme left.''
In British general election contests, victory usually goes to the party which successfully captures the middle ground. It is this that Labour Party leaders Foot and Healey are determined to do.
But they fear that Mr. Benn and those who support his left-wing approach will appeal only to the radical strand in the Labour Party, leaving the Conservatives and the Liberal-Social Democratic Alliance to gather votes from moderate voters, including trade unionists.
The latest public opinion polls offer little comfort to Mr. Foot. A poll carried out for the BBC last weekend showed 45 percent proposing to vote for Mrs. Thatcher's Tories, compared to 34 percent for the Labour Party and 20 percent for the Liberal-Social Democratic Alliance.
The same poll showed that if Mr. Healey replaced Mr. Foot as party leader, 42 percent would vote Tory, the same percentage Labour, and 15 percent the Alliance. The chances of Mr. Healey moving into Mr. Foot's place as party leader before the election, however, are virtually nil.
Labour Party leaders say last week's local government election results in England and Wales revealed a much narrower gap between Tories and Labour than the opinion polls suggest. Nationwide, the local government voting gap was only 5 percent, compared to 11 percent in the opinion polls.
An irony for Mr. Benn is that because of boundary changes his old Bristol constituency has disappeared. Another seat which he has agreed to contest is rated highly marginal. Unless there is a significant swing to Labour - a swing which Mr. Benn's more moderate party colleagues say is less likely due to Bennite tactics - he may find himself without a place in Parliament after June 9 .