PARIS – French voters appear ready to deliver a humiliating rebuff to the European establishment by rejecting the proposed E.U. constitution, according to opinion polls yesterday.

They showed the “No” vote strengthening to an eight-point lead in the final days of campaigning before Sunday’s referendum.

With objectors gaining ground in France and firmly entrenched in Holland, where a referendum is to be held three days later, Brussels is considering how to deal with the previously unthinkable combination of two founder members of the European Union turning against the blueprint for further integration.

A French or Dutch objection could defeat the treaty outright or delay it for a number of years, frustrating its stated goal of streamlining decision-making in the expanded 25-member union.

Each country must approve the constitution for it to take effect next year.

A negative vote could bring months of political confusion and economic instability amid recriminations and debates over the next move.

A majority of Dutch respondents, who vote on June 1, are also against the constitution.

The Dutch prime minister, Jan Peter Balkenende, appealed to French and Dutch voters yesterday to shun the appeals from the increasingly confident “No” campaigners.

Speaking to reporters in The Hague, he refused to speculate about the chance of a “No” vote.

“I have experienced so often as prime minister that polls are different than the final outcome. Next week is the real moment,” he said. “Everybody’s vote counts. The question is whether we go ahead with Europe or stand still.”

In France the main focus of opposition is among voters of the left, who see it as a British blueprint for a free-market Europe, which would sweep away the French social model.

The former socialist prime minister who is defying his party’s decision to join President Chirac in a “Yes” vote, Laurent Fabius, said: “It is important that the ‘No’ vote be from the left, a Socialist no, a socially-conscious no.”

Voters in France, despite its role at the heart of the European project, have expressed their alarm at the prospect of membership talks with Turkey and a proposal to allow E.U. countries with low labor costs to provide services in richer states.

They have also used the referendum as an opportunity to express dissatisfaction with the economy under Mr. Chirac, who faces a major humiliation on Sunday.

A rejection by French voters would give Tony Blair an excuse to cancel a British referendum. It would also saddle the incoming British E.U. presidency with the task of deciding what to do with an unwanted constitution.

Britain’s Foreign Secretary, Jack Straw, insisted yesterday that the Government would press ahead with a referendum on the constitution as long as there is a treaty to vote on.

The Liberal Democrat deputy leader, Sir Menzies Campbell, said there would be no point in holding a UK referendum if France and the Netherlands rejected the treaty.

Mr. Chirac’s best hope of avoiding defeat lies in a last-minute rally in support of the constitution among undecided voters, who number more than half in some polls.