Britain has received its first cruise missiles from the United States, but Prime Minister Margaret Thatcher believes their deployment will not necessarily prevent progress in arms control talks between the Americans and the Soviets at Geneva.
Hours after a huge Starlifter aircraft delivered the missiles to the Greenham Common base in rural Berkshire, antinuclear protesters announced plans to keep them ''locked in'' there, preventing their deployment.
But Mrs. Thatcher, addressing the Lord Mayor's banquet in the City of London, insisted the missiles were crucial to the nuclear defense of Western Europe. They were also, she declared, a possible stimulus to realistic nuclear arms negotiations with Moscow.
Arrival of the missiles caught nuclear protesters by surprise. They had believed the first shipment would arrive later in the week.
The Labour opposition suggested the missiles had arrived earlier than the government expected. But it appeared that an official attempt to mislead the protest movement about the true arrival date had proved successful.
British authorities show every sign of taking seriously the protesters' threat to bottle up the cruise missiles in their Berkshire base by impeding the movement of the tracked vehicles that are meant to carry them around the countryside to secret firing locations.
Defense Secretary Michael Heseltine caused an outcry in the House of Commons by threatening that protesters who tried to penetrate the Greenham Common storage silos ran the risk of being shot. Many of the people objecting to the missiles are women.
It is believed that when the time arrives for trial deployment of cruises beyond Greenham Common, extraordinary security measures will be enforced, with police and Army cooperating to avoid attempts by protesters to disrupt the movement of the missiles.
Mrs. Thatcher is taking the firmest of lines about her government's decision to accept the cruise missile on British soil. Even since the US invasion of Grenada, she has insisted that her government trusts the Americans over the deployment of the missiles. But she has not been able to avoid bitter exchanges with political opponents.
When he learned the first missiles had arrived in Britain, Labour opposition leader Neil Kinnock said the weapon would open the British countryside to the threat of saturation bombing by the Soviet Union. He said the missiles should go back to the US as rapidly as possible.
Mrs. Thatcher flatly refused, suggesting the weapons could go back to America if and when the Soviet Union decided to disband its force of SS-20 missiles. She told the House of Commons the British deployment would take five years, giving the Soviets plenty of opportunity to reduce the SS-20 force.
Despite considerable disquiet not only in Parliament but also among the general public over arrangements for the control and possible firing of cruises, Mrs. Thatcher has flatly rejected demands for a ''dual key'' arrangement giving Britain and the US joint control.
She has also attempted to soften her anti-Soviet attitude in some respects. On the day the arrival of the first missiles was announced, Mrs. Thatcher revealed plans for an official visit to Hungary next year. This, she said, was a sign of her continuing interest in detente.
For the protest movement, the arrival of the first batch of missiles in Britain constitutes a turning point in their campaign. The Rev. Bruce Kent, general secretary of the Campaign for Nuclear Disarmament, said the main effort now would be to mobilize public opinion to force the government to have the missiles removed.
But the Thatcher government has a huge majority in the House of Commons and has firmly nailed its defense flag to the cruise mast.