The people of Ireland now have the fate of Britain in their hands. The deal reached by Irish Premier Brian Cowen at the EU summit at the weekend allows him to call a new referendum on the Lisbon Treaty in early October.

If the Irish vote "yes," and Gordon Brown's wretched lame duck of a government clings to power in London until next year, then Britain will probably remain a member of the EU. If the Irish vote "no," then Britain's disengagement is likely to begin.

David Cameron, the Conservative leader who looks almost certain to win Britain's next election, has promised a separate British referendum on the Lisbon Treaty if the ratification process is not completed in all 27 EU member states. The Irish referendum is one such hurdle. The German Constitutional Court is another, but on what seems to be a technical matter. The Czech and Polish presidents have yet to sign the ratified document, but all other legal procedures are complete.

So an Irish "yes" vote means that Cameron will probably not have to carry out his promise to hold a referendum in Britain. This would probably come as something of a relief to Cameron, who is happy to run against the EU when campaigning in Britain, but does not want to leave the EU altogether. The question will be how Britain can stay a member of an organization when all its other members have agreed to create an entirely new one, and Britain alone rejects it.

Most opinion polls say the British would vote against the Lisbon Treaty by a margin of two to one. In the European Parliament elections earlier this month, Gordon Brown's Labor candidates were beaten into third place behind the United Kingdom Independence Party, a single-issue party that says "no" to Europe.

Most Conservatives now oppose the treaty, even though it was their party that took Britain into Europe more than 30 years ago, and many of their senior figures remain fundamentally pro-European. These party splits over Europe have been deeply damaging. Prime Minister Margaret Thatcher was toppled for being too skeptical of Europe. Her successor John Major was emasculated in Parliament by his own anti-Europe rebels. The arguments over the treaty could bring back those bitter divisions of the past.

"The treaty transfers power from national parliaments to the Brussels bureaucracy and would make Britain a province of a European state with a European president," comments William Rees-Mogg, a former editor in chief of London's The Times who speaks for many traditional conservatives.

So David Cameron's priority is to ensure that Europe does not become so prominent an issue that it splits his party yet again. That was why he promised the referendum, in what at the time seemed like an empty gesture. Now it could be the issue that defines his political future.

This issue has been building for years. The Lisbon Treaty, which gives the EU a powerful new president and foreign minister and gives greater powers to the elected European Parliament, originally began 10 years ago as a new EU Constitution.

When the French and Dutch voters rejected that idea in a referendum in 2005, the Eurocrats of Brussels cunningly reinvented the constitution as a treaty. This meant it could be ratified by parliaments without need for a referendum -- except in Ireland, where the government was not allowed to get away with such fancy footwork.

It is largely the same document, although some symbols that suggest a European federal state have been removed, like the EU anthem and flag.

The next Irish referendum is now the key. At the last time of asking, they voted "no." But since the rest of the EU has now agreed to add a solemn protocol to the treaty to guarantee that the Irish will not have to give up their traditional neutrality and their control over the own taxes and abortion laws, Cowen can now tell his voters that he has secured a better deal than the one they rejected.

The big difference is that when they voted "no," the Irish thought their economy was doing fine. This year their economy is in deep recession, and without the euro and the prospect of EU support, the country would be at great risk of defaulting on its debts. So the opinion polls suggest that the Irish will vote "yes" in October and put the British political system into an agonizing dilemma.

Cameron would love to replace Gordon Brown sooner rather than later, but not if the timing would force him to hold a British referendum on Europe. Brown would rather hang on until May 2010, the final date by which he must call an election. But if Brown can be toppled soon by his own party, and a new leader calls an election in October, the opinion polls say they would lose many fewer seats in Parliament. And they would thrust Cameron into an immediate and ruinous crisis over Europe.