Campaigning in favour of Europe is "not an immediate issue", the prime minister has said.

Speaking at his monthly press conference on Monday, Tony Blair told reporters that it was "sensible" for the European Union to adopt a constitution.

And he insisted that Britain's "red lines" in key areas such as foreign and defence policy had been maintained.

But he was forced onto the defensive by media reports that the government was surrendering its veto on asylum and immigration issues.

The latest row comes as the prime minister prepares to put his signature to the constitution at a gathering of the EU's 25 leaders in Rome this Friday.

Meanwhile, it has also been reported that ministers are considering a referendum on the constitution in the first half of 2006, after Britain's presidency of the EU finishes.

But Blair played down suggestions that a date has been fixed, saying that "there are still a lot of stages that have to be gone through before we get to that referendum".

Countdown begins

When the constitution is adopted this week, European states will have two years in which to ratify it.

And with the clock ticking, Blair was pressed on whether he should be leading a more active pro-European campaign.

In a lukewarm response, the prime minister said he was "explaining it to people but it's not an immediate issue for all the reasons that I've explained".

He had earlier defended the details of the constitution.

"We will continue to make the case for a set of rules," Blair said.

"Europe is expanding to 25. In the next few years we will in all likelihood, if they meet the criteria, be taking Turkey into the European Union and other countries [such as] Bulgaria and Romania.

"It is sensible that we have a set of rules which allow the European Union to work more effectively."

The prime minister said the "key thing for the public is to understand that tax, foreign policy, defence and - because of the opt ins that we have - asylum and immigration continue to be decided in this country".

"Where we want to cooperate with the rest of Europe we are entitled to do it, and some times its very sensible to cooperate with the rest of Europe, but we are not obliged to in those areas," he said.

"I've made that case continually and I've no doubt I'll be making it again on Friday but there are still a lot of stages that have to be gone through before we get to that referendum."

Making the case

Reacting to the comments, Britain in Europe said it remained confident the government will throw its weight behind the constitution ahead of the referendum.

"This will be a campaign of many stages and will require an appropriate effort from people from all parties and none," said campaign director Lucy Powell.

"The government clearly has a role to play and we are confident they will be making the case strongly as the treaty makes its way through parliament in the months ahead."