Britain said today that its latest Falkland Islands casualties, believed to be the worst British death toll in a single day since the fighting began, would not delay its drive to retake the islands' capital of Stanley.

Defense Minister John Nott told the House of Commons that the death toll from an Argentine air attack on two British landing ships Tuesday was being withheld because its release "could be of assistance to the enemy and put our own men at greater risk." It was the first time the government has imposed a news blackout on casualty figures.

Argentina said only British stragglers remained at Fitzroy, which a military spokesman reported was bombed again yesterday. Argentina claimed its Air Force also bombed British positions on Mt. Kent and said two British Harrier jets were shot down. Story on page A25.

Nott denied that Tuesday's losses of men and materiel had set back the timetable for the planned final assault on Stanley, the last remaining Argentine military stronghold on East Falkland Island. He told Parliament that "the task force commander's plans have not been prejudiced by these attacks, and the losses of stores and equipment are already being made good from other stocks held ashore."

Prime Minister Margaret Thatcher told reporters in Bonn, where she was attending the summit conference of the North Atlantic Treaty Organization, that the figures on British deaths were being withheld because the government had not yet been able to compile an accurate body count. She said there were "a considerable number of casualties."

Nott revealed that four Royal Marines and two seamen were killed in a separate air raid Tuesday on an unnamed landing vessel. A fourth ship, the frigate Plymouth, also was hit. Nott said five men were injured, but the ship remained operational.

One report in the British press said at least 20 men were killed and 80 wounded aboard the landing ship Sir Galahad, and another said that 39 members of the Galahad's 68-man crew were missing. It has been estimated that at least 250 British troops were being ferried to shore aboard the Galahad in broad daylight at the time of the attack. The six dead announced today bring to 142 the total officially confirmed British deaths since Argentine forces seized the Falklands on April 2.

London has been slow to release any information on the air raids, the first major Argentine assault in nearly two weeks. When it first announced the attacks Tuesday night, the Defense Ministry emphasized Britain's success in landing ground forces south of Stanley at Fitzroy and Bluff Cove. The ministry's statement mentioned simply "some damage" to three vessels and officials told reporters that the term indicated the damage had not been major. It was only last night, after dispatches from British journalists in the Falklands indicated that casualities were extensive, that the ministry conceded "heavier losses."

There was little criticism in Parliament of the blackout on casualty figures. But Labor Party leader Michael Foot told Parliament the deaths were a warning that "if the fighting continues to the bitter end, many more lives will be lost on both sides." Foot urged Prime Minister Thatcher to return to the U.N. Security Council with a fresh proposal for a cease-fire coupled with an Argentine military withdrawal from the islands.

Deputy Prime Minister William Whitelaw, speaking for the absent Thatcher, rejected Foot's plea, saying it was up to the Argentines to end hostilities by withdrawing their remaining forces.

Meanwhile, the Foreign Office announced that Argentine Capt. Alfredo Astiz, commander of the defeated Argentine garrison at South Georgia Island, had been released and was returning to Buenos Aires.

Astiz, suspected by French and Swedish authorities of involvement in the disappearance of three of their citizens in Argentina, had been brought to Britain for questioning. The Foreign Office said he was interviewed Tuesday by a British police official, but refused to answer any questions, including those submitted by France and Sweden.

A Foreign Office spokesman said Astiz had been released in accordance with the Geneva Convention's rules on prisoners of war.