Neil Kinnock will warn today that the referendum on the proposed European Union constitution will determine whether Britain is "really in or really out" of the EU.
The European Commissioner will tell the Welsh TUC conference in Llandudno that Britain will be relegated to an "outer core" of EU members if it rejects the constitution in the referendum expected in the autumn of next year.
"No one should fail to understand the grave reality that our people will really be voting yes or no to our country' s engagement in the EU," he will say. "A no vote means rejecting that and instructing a British Government to veto the treaty. The resulting paralysis would be unsustainable."
The former Labour leader, who has criticised Tony Blair's decision to promise a referendum, will add: "In essence, two European Unions would be created, an inner-core majority of countries that could and would proceed with developing internal and external economic and political policies, and a minority outer group of no-voting countries that, because of basic market realities, had to live with those policies.
"A no-voting UK would be in that outer group. That is what the no campaigners plainly want. Partial withdrawal without total isolation is their aim."
Dismissing Tory plans to "renegotiate" Britain's relationship with Europe, Mr Kinnock warned that other EU members would not "calmly continue with business as usual when UK relations with most of the EU have been put into a perpetual state of instability".
He will say Tories and Eurosceptic newspapers should stop "trying to fool people into thinking a no to the constitutional treaty would somehow protect democracy, sovereignty, jobs, opportunities, investment and influence.
"It would sabotage all of those. What kind of patriotism is that? It looks more like a sell-out of national interests".
Valery Giscard d'Estaing, the former French president who helped draft the constitution, said yesterday that Britain would not be outside the EU if it votes no but would be pushed to its "edge".
He told BBC Radio 4: "If finally the British said no and the other Europeans said, 'We want to go [ahead]' they will have to find an accommodation ... and it is true in that case Britain will not be in the core of the system but at the margin of the system.
"It is not a question of saying yes or no to Europe. It is a question of making Europe function.Of course, if someone says 'we do not accept the way that Europe functions' it will have to assume the consequences of its own choice."
- More about: