Tony Blair has acknowledged he faces "a big battle" to persuade Britain of
the merits of the planned constitution for Europe.
The EU is expanding
His comments come as a poll suggests that more than half of voters believe Britain should quit the European Union, rather than accept a constitution which transfers more powers from Westminster to Brussels.
Conservative shadow home secretary Oliver Letwin on Sunday said the draft new constitution was "slippery slope" towards a "pan-European
system of criminal justice".
European judges would have unprecedented
power to fix sentences under the plans, said Mr Letwin.
The prime minister told Sky News: "There's a big battle
going on in Europe at the moment, but I look forward to it.
"This is a fundamental question for this country - do we want to be part of
the main strategic alliance right on our doorstep - Europe?
"Do we really want to be partners, play our full part in it, make sure that
it is shaped in the British national interest, or do we want to be marginalised
and left without influence?"
Anyone in Britain who claims the constitution will not change
things is trying to sweeten the pill for those who don't want to see a bigger role for Europe
Former Italian PM Lamberto Dini
According to a YouGov poll in the Mail on Sunday, 51% of Britons said withdrawal from the EU was preferable to the "surrender" of further powers to Brussels.
That was against 29% who said Britain should accept a loss of power in order to stay in the union.
The survey also suggested 75% of voters wanted a
referendum on the draft constitution.
The Conservative party has repeatedly called for a referendum on the convention.
But Mr Blair has rejected the idea of a referendum, accusing those demanding one of secretly wanting total British withdrawal from the EU.
Mr Letwin's concerns came in a letter to Home Secretary David Blunkett.
constitution represents a real challenge to control of criminal justice by our
Parliament in our country," he wrote.
The Tory frontbencher argued the constitution could see the UK's adversarial style of court trial replaced with the inquisitorial methods used in other European nations.
And he said the draft made clear the UK would lose control of its asylum policy.
The government insists the UK will keep the opt-out it secured at the Amsterdam summit on common border and asylum controls.
On Sunday a former Italian prime minister, who now sits on the body which designed the planned constitution, warned the UK public of being misled over the draft constitution for the European Union.
"Anyone in Britain who claims the constitution will not change things is trying to sweeten the pill for those who don't want to see
a bigger role for Europe," Lamberto Dini told The Sunday Telegraph.
"The constitution is not just an intellectual exercise. It will
quickly change people's lives...
"Eventually the union will be able to make legislation of its own. It will become an institution and organisation in its own right."
His comments appeared to echo the stance of the Conservatives, who argued on Saturday that the new constitution would give Europe the characteristics of a state.
"The Europe which is now on offer from the Convention, which when you add all the bits together is actually the creation of effectively a new country," shadow foreign secretary Michael Ancram said.
But the government's chief negotiator on the constitution, Peter Hain, said the new framework stipulated that the EU would stay a partnership of nation states.