LONDON, Nov 23 (Reuters) - Prime Minister Tony Blair said on Friday that Britain could no longer lag in Europe, sparking speculation the push is now on for euro membership.

In a markedly pro-European speech, Blair outlined the case for closer ties with the continent and ran through the missed opportunities of decades past.

"Britain's future is inextricably linked with Europe," Blair said. "We must be whole-hearted, not half-hearted, partners."

The Labour prime minister reaffirmed his intention to give the British public the final say on joining the European single currency, saying the economic conditions must first be right.

But Blair's clearest signal was that Britain must engage -- or risk falling behind its partners across the water.

"We will not have influence if we only ever see Europe as in opposition to Britain," he told a nation that remains largely sceptical about the wisdom of swapping the pound for the euro.

The euro will replace national currencies in a dozen EU nations on January 1. Britain is not one and financial markets are on alert for any signal about when it might take the plunge.

The pound fell to four-week lows against the euro as the market took advance leaks of the speech as a sign of greater commitment to the euro. It edged back up after it was delivered.

Blair carefully restated his wait-and-see euro policy: to hold a referendum only if the government judges it to be in Britain's economic interest to join the currency club.

But every other phrase about the broader European project was strikingly positive. Campaigners on both sides of the fence believe Blair has started a quiet campaign for euro entry and pundits swiftly started speculating about dates.

"Read between the lines and the message is clear. Britain's decision to join the euro is inevitable," said Simon Murphy, leader of Blair's Labour Party European MPs in Brussels.

The greatest hurdle to membership could yet be Blair's powerful finance minister, Gordon Brown, long rumoured to be more cautious about the euro and jealous guardian of the economic assessment the government has promised by mid-2003.

A spokesman for the No Euro campaign said "insiders" had told it Brown "is now hinting privately that he could support the prime minister's desire for an early referendum".

That matches a Guardian newspaper story which said Brown had told colleagues he wants to join the euro. A Treasury source dismissed the report as "complete and utter nonsense".

Blair walked his audience -- academics at the European Research Institute in the midlands city of Birmingham -- through Britain's rocky post-war relationship with the EU.

It had been a "history of missed opportunities", the greatest of which was failing to join the bloc at the start.

"Politicians of both parties have consistently failed, not just in the 1950s but on up to the present day, to appreciate the emerging reality of European integration," he said.

"In doing so they have failed Britain's interests."

The Sun tabloid ran a front page story saying Blair had "privately signalled" a referendum would be held at the same time as the next election, expected in 2005.

Minister for Europe Peter Hain said no date had been set.

But Hain said the introduction of euro notes and coins in the new year could have an impact on public opinion, particularly after Britons holiday abroad next summer.

"They will have a chance to make up their own minds rather than rely on...tabloid imagination and fear," he said.

Most experts say if Blair wants to join the euro before the next election, the latest a referendum can be held is in 2003.

The government says it would take at least two years after a "yes" vote to prepare society and commerce for a new currency. But polls show that up to 70 percent of Britons want to keep the pound, and some hard campaigning would have to be done first. (Additional reporting by Kate Kelland, Steve Cunningham, Carolyn Cohn) Nampa-Reuters