WITH six weeks to go before the European Community's heads of government meet in Maastricht, Netherlands, Prime Minister John Major has hardened Britain's stance on key European policy issues.He is trying to head off two threats to his negotiating position at the Dec. 9-10 EC summit on political and economic union. One threat comes from EC countries and the Brussels Commission, all eager for Britain to commit itself to closer European integration, and the other from right-wing members of Mr. Major's ruling Conservative Party, pledged to resist integration, who are threatening a House of Commons revolt before the summit. The prime minister also is letting political factors move to the center of his calculations as opinion polls show the opposition Labour Party opening up a significant lead over the government. For the first time Major's senior ministers are talking about the possibility of failure at Maastricht. They are openly condemning what they say is EC interference in British affairs. The new line, spelled out on Sunday by Douglas Hurd, Major's foreign secretary, surfaced as the Netherlands, current president of the EC, prepared to unveil revised draft proposals on EC political and economic union designed to smooth the way for the signing of a treaty at Maastricht. Signaling a more obdurate British position, Mr. Hurd indicated that at Maastricht Britain would decline to sign a treaty on union if it was unhappy with the text. A decision to hold another summit next year, under Portuguese chairmanship, "cannot be ruled out," Hurd said. It might be needed to settle matters left unresolved at Maastricht. Britain was still hoping for agreement, but there might be "circumstances in which people press federalist ideas that the prime minister simply won't be able to recommend to the Cabinet and the House of Commons," Hurd added. A senior Conservative official, interpreting Hurd's comments, said the Major government was increasingly worried that with a general election only a few months away the prime minister could not afford to sign anything that might split his party and be rejected by British voters. A more rugged British approach to relations with the EC was essential, the official said. Until now, the government has preferred to stress that it wants Britain to be at the heart of Europe and that Britain would be looking for agreement at Maastricht. It has avoided suggestions that Major might be willing to veto an agreement. As Major prepared for a series of meetings with EC leaders (he will see Germany's Chancellor Helmut Kohl on Friday) there were indications that he might have his work cut out heading off a right-wing revolt. Norman Tebbit, a former Conservative Party chairman and now a leading skeptic of integration, over the weekend urged Major to give a flat British "no" to any EC move at Maastricht to adopt a single European currency. Mr. Tebbit speaks for an influential wing of his party, and his views are close to those of Margaret Thatcher, the former prime minister. Mrs. Thatcher has threatened to make her views plain if she is dissatisfied with any document Major signs at Maastricht. In a bid to strengthen his negotiating position in the run-up to Maastricht, and to close Conservative Party ranks, Major plans to stage a parliamentary vote on European policy on the eve of the summit. A Conservative parliamentary source said the prime minister was looking for "convincing endorsement of his policies by Conservative MPs." The source said Major was hoping Thatcher, Tebbit, and their supporters would decide in the end not to challenge his handling of the EC unity talks, in order to preserve Conservative Party electoral prospects. Hurd's comments about Maastricht coincided with an opinion poll, published by the respected Mori organization, showing that the opposition Labour Party had opened up a six point lead over the Conservatives. Such a lead would give Labour a comfortable parliamentary majority in a general election. The prime minister's close associates say his own mood about the coming negotiations has been soured by a series of open clashes in the last two weeks between British ministers and officials of the European Commission in Brussels. Last week Major sent a sharply worded rebuke to Jacques Delors, president of the Commission, rejecting an attempt by Brussels bureaucrats to force a halt to seven major British construction projects, including a costly rail link between the Channel Tunnel and London. Carlo Ripa di Meana, the EC environment commissioner, claimed Britain had failed to carry out proper environmental impact studies on the seven projects. In his letter to Mr. Delors, Major rejected the commissioner's objections, which had been accompanied by a threat to take Britain to the European Court if it fails to comply. Such matters lay beyond the competence of Commission officials, the letter said. Another Brussels Commission plan seeks to impose on Britain, along with other EC countries, a ban against employees working longer than 48 hours a week. It too drew British ire. The proposal is contained in a draft directive soon to be considered by the EC council of ministers. A senior government minister described the directive as "another example of Brussels trying to stick its nose into our affairs."