In a country that takes its soccer very seriously, many fans here are growing increasingly worried about a different type of casualty that may result from the conflict over the Falkland Islands--British participation in the final rounds of the World Cup.

Officials both here and in Spain, where the tournament begins June 13, are saying that Britain's three squads--England, Scotland and Northern Ireland--will compete in the 24-team tournament, which includes the world-champion Argentinians. Publicly, soccer association officers are going ahead with preparations, trimming their squads, scouting first-round opponents and making optimistic statements about their prospects on the field.

Behind all that, however, is the threat of faltering negotiations and worsening hostilities between Britain and Argentina--and the possibility that Prime Minister Margaret Thatcher's government will step in and ask the players to stay home. The cabinet has not made any decisions, but spokesmen say that the situation is "under daily review."

If past events are any guide, the Thatcher government is likely to come down on the side of withdrawal. The government strongly backed the boycott of the 1980 Moscow Olympics following the Soviet invasion of Afghanistan, although many of the top British athletes still competed in the Games.

In theory, the British teams have until June 4 to submit player lists to international soccer authorities. But as opening day nears and uncertainty grows, anxious World Cup organizers have demanded an immediate response so that, should the British pull out, substitute teams can be arranged.

There is no chance of the powerful Argentinians pulling out of the competition.

The British, meanwhile, are reluctant to withdraw while negotiations on the Falklands continue. "It will definitely be a last-minute decision," said one source privy to discussions in the ministry of sport. "The cabinet members don't even know in their own minds what they are planning to do."

Most observers here agree that a decision to boycott would not go down well with the British public. Soccer is something of a national passion.

Although none of the three British teams stands a good chance of winning the cup, it is the act of participation that matters most to their fans. "It goes beyond football," Nicholas Keith, sports editor of the London Times, remarked. "It's a huge international event. There's a national feeling that the games should be allowed to be played just as the Pope should still come and visit."

Opposing this attitude is a body of opinion that contends that British athletes should not be playing games with citizens of a country that they are fighting. England, if it won its group, would play Argentina, if it won its respective group.

"If the serious shooting begins, if the blood begins to flow," the Manchester Guardian commented in an editorial this week, "few will have much time or stomach for World Cup football--the players included."

Although the cabinet cannot force the soccer associations and players to abide by its desires, the clubs and some of England's top players, including striker Kevin Keegan, have pledged that they will go along with any requests from 10 Downing Street.

But such a decision would not come easily. Harry Cavan, president of the Northern Irish Football Association, says that, "In spite of the noises that have been made, the players would be extremely disappointed, both in terms of national honor and for financial reasons."

Should they withdraw, each of the teams would face an immediate penalty (levied by the International Football Association, FIFA) of about $8,000. On top of that would come personal losses and demands for compensation from the Spanish organizers. "Unless there was direct government intervention--and they were prepared to indemnify the football associations--then I cannot see any possibility of a withdrawal," Cavan said.

But Britain's dispute in the South Atlantic has already resulted in the cancellation of a trip by a Republic of Ireland soccer team to Argentina to take on the world champions.