London — Britain's problems with its allies over the Falklands crisis appear to be intensifying.
At the outset, European allies surprised the British government with their readiness to back a policy of economic sanctions against Argentina. The main problem at the time was the United States, which for a full month held to a policy of ''evenhandedness'' in its dealings with London and Buenos Aires.
Now, nearly seven weeks after President Leopoldo Galtieri ordered his troops onto the Falkland Islands, that situation has reversed itself.
The United States is officially ''off the fence'' and helping Britain. The Europeans are increasingly critical of Margaret Thatcher's handling of the crisis and nervous about the likelihood of a wider war.
European disenchantment began in the Republic of Ireland, where Prime Minister Charles J. Haughey decided that automatic endorsement of sanctions against Argentina was not possible for his government.
Then the Italian parliament declared itself against reendorsement. Denmark affirmed that it would take a position of its own. This week, meeting in Luxembourg, the 10-member European Community (EC) decided to renew sanctions only for seven days instead of the four weeks Britain had asked for.
The European decision came as a double diplomatic blow to Britain. In part, it suggested that support for the British position in the Falklands crisis was eroding.
Even more awkward for Mrs. Thatcher and her ministers was the readiness of the Europeans to link their attitude to the Falklands crisis to another key issue: Britain's refusal to go along with increased farm prices within the Ten.
No one was saying so publicly, but British obduracy on farm prices caused a number of European governments to alter their stance on the Falklands.
Within the Thatcher Cabinet, the change of mood in Brussels did little to precipitate second thoughts on Falklands policy. On the contrary: The seven-day deadline for renewed European sanctions strengthened the hand of those favoring an early military strike against Argentina.
Also, the solidarity of NATO behind Britain was some consolation to Mrs. Thatcher. Privately, however, strains have begun to appear in NATO as well, particularly because of the large numbers of ships and aircraft currently committed to the South Atlantic. In normal times they would be at the disposal of NATO commanders.
The negative approach of the Irish government has been causing considerable anger in Whitehall. Prime Minister Haughey is a hard-liner on the problem in Northern Ireland. Now he has come out openly against the military option in the Falklands dispute.
British officials concerned with the Ulster problem say it will take a long time for relations between Dublin and London to be restored to cordiality.
Some of Mrs. Thatcher's advisers are pointing out that it was inevitable, given the lengthy peace negotiations, that allied solidarity would fray.
What is needed, these advisers are saying, is decisive military action by Britain. A determined and successful strike against Argentine positions on the Falkland Islands would greatly strengthen Britain's international stance, according to this advice.
Much the same applies to the United States and its relationship to Britain. The longer fruitless negotiations continue, some top advisers are saying, the more likely it will be that President Reagan begins to ''wobble'' in his support for Britain.
By late this week, according to defense analysts, the main elements of the Royal Navy task force will be in place and at optimum readiness for a sea and air assault.
As international support for the British position weakens, the imperatives of the military option appear to be strengthening, tilting Mrs. Thatcher toward the possible use of more force to break the diplomatic deadlock and repossess the islands.