After 12 hours of marathon negotiations, the leaders of the nine Common Market countries again failed to agree on a settlement of Britain's complaint that it pays far too much into the European Community's complicated budget system.

Despite determination that the European Community should speak with a single voice, the Common Market remains split on important budget and trade policies that could seriously undermine its unity and original purpose as a trading community.

British Prime Minister Margaret Thatcher refused a number of compromise offers today to reduce Britain's financial contribution, which the British say is far too large for one of the Common Market's poorest members. bShe rejected a final offer to reduce the annual British net contribution from more than $2 billion to lees than $500 million because it would only cover the next two budget years.

She also refused to agree to an increase in Common Market farm price supports and changes in its fishing rights policy that the French and other government sought to link with a settlement of the British budget claim. o

Once again, Thatcher's stubborness dismayed the other European leaders at the two-day Common Market summit here. But her opponents acknowledged that she gave more ground than at last winter's summit in Dublin that also ended in a stalemate on the budget issue.

Nevertheless, French President Valery Giscard d'Estaing told reporters tonight that the vote on almost every issue in today's negotiations was 8 to 1, with Thatcher the hold out vote. He said this showed that Britain cannot satisfy its budget complaints within the present community framework, a reference to early French hints that British should withdraw to associate membership in the Common Market.

West German officials said Thatcher turned down more than a dozen different offers that would have cost West Germany, the Common Market's richest member, considerable more money to reduce what Britain would owe.

Giscard was one of several European leaders who said they did not want to face the issue again at the next Common Market summit in mid-June in Venice. But Thatcher said the agenda was controlled by Italy, which currently holds the Common Market presidency and that Britain will seek further budget negotiations.

The impasse could lead to retribution by both sides. Giscard said he would raise French farm prices despite Thatcher's refusal to agree to the new support levels. Thatcher has threatened in the past to withhold British tax contributions to the community budget and block Common Market business at the meetings if its net contribution is not adjusted to her satisfaction.