Former Defense Minister Michael Heseltine on Tuesday forced Prime Minister Margaret Thatcher into a second-round ballot in their struggle for leadership of the Conservative Party.

The vote was a sharp setback to Thatcher, who has governed Britain for 11 1/2 years. If Thatcher loses the leadership of the party, she would step down as prime minister.Thatcher won 204 votes from Conservative Party members of the House of Commons and Heseltine received 152 votes.

That gave Thatcher a lead of 9.7 percent. She needed a 15 percent advantage to win outright.

Thatcher had been the unchallenged leader of the Conservative majorities in Parliament.

In Paris, a calm Thatcher said she would fight on to a second round.

The second-round vote on Nov. 27 will be open to additional candidates. Foreign Secretary Douglas Hurd has indicated he might run against Heseltine if Thatcher dropped out.

The prime minister learned of the result at the British Embassy in Paris, where she was attending the 34-nation summit on European security.

Thatcher had been at pains to keep up business as usual - although the business last weekend included a round of lengthy interviews with newspapers about the leadership challenge.

Thatcher, 65, had argued that Hesel-tine's economic policies were a betrayal of her free-market crusade.