The government's failure to come out in clear support of euro membership could rule out a referendum in this parliament, Charles Kennedy has warned.

The Liberal Democrat leader said that Tony Blair is failing to apply the lessons of his own "campaigning handbook" to the fight to take Britain into the single currency.

In an interview with ePolitix.com, conducted before Kennedy departed for his Far East honeymoon, he warns that the decision on how to proceed rests with just two people - Gordon Brown and the prime minister.

"I suspect that they have not yet - either individually or collectively - made up their own minds, and I don't think they will do so for some time. The two of them - at some point - have to have that critical internal conversation. It's pretty clear it hasn't happened yet and I don't know when it will," he said.

Kennedy believes that while opinion polls show hostility to the euro, "most people accept that the time will come when we can no longer be insular".

"The other problem is that the prime minister is in danger of ignoring his own campaigning handbook - that you can't win elections in two weeks, you win them in two years. It's the same with a referendum campaign.

"With an issue which is as complex and controversial as this, you just can't expect to win it from a standing start in six weeks. You should be making the case week, upon week, upon week - and making it now," he told ePolitix.com.

And in a stark warning about the implications of failing to mount a serious campaign in favour of the euro, he warns that the prime minister will become increasingly likely to abandon any plans to hold a referendum before the next general election.

"There will only be a referendum when Gordon Brown and Tony Blair are convinced they can win it. They are not going to hold a referendum if they think they will lose - or both their careers will be in considerable trouble.

"So the referendum will only take place if they believe that there is, within any acceptable democratic margin of error, a high probability of success.

"The irony is that the longer they prevaricate about making the case, the more difficult it is to achieve. So the whole thing becomes self fulfilling.

"Then, before you know where you are, we will be half way through this parliament and it will be too late to hold a referendum this side of a general election. That's the danger zone - and we're heading right into it."

The Lib Dem leader also says that the government's failure to give a "much clearer lead in Europe" could undermine the work of Britain in Europe, the campaigning organisation expected to play a key role during any referendum campaign.

"I think the practical difficulty for Britain in Europe is that there is no substantive campaign to fight and it's difficult to have a campaigning organisation without a campaign," said Kennedy.

He is also critical of the No Campaign's adverts urging the public not to back the euro, condemning their "rather nasty populist" approach.

"There were certainly people who were offended by the Hitler analogies. If this is the opening shot of the campaign, I hate to think - when we get into the campaign proper - what they will come up with. It will backfire badly on them."