Former minister Stephen Byers has said the government must put "far more effort" into promoting the benefits of EU membership for Britain.
Mr Byers wants the advantages of EU membership emphasised
Mr Byers urged Labour to have a more "patriotic" approach to the EU constitution and argue for the treaty.
"Labour must stop being defensive about Europe," he said.
Former Conservative deputy prime minister Lord Heseltine backed Mr Byers' calls, saying the UK should have a leading role in Europe.
Mr Byers, a former Labour transport minister, said the Conservatives should not be allowed to persuade people Europe was a conspiracy against them.
He said jobs depended on making the case.
"We will need to demonstrate there is a patriotic case for Europe and the new treaty in that Britain and its people would stand to gain from the process," the former Labour minister said speaking ahead of a European conference in London.
He added Britain should not fail in its responsibilities towards the rest of Europe by rejecting the constitution.
"At no point in our long history has Britain ever been prepared to deny our responsibilities and interests in Europe's future.
"We must not let it happen now," he said.
Speaking to the BBC's Today programme, Lord Heseltine said he had huge pride in the UK and believed it should play a leading role.
Without Britain's interests in the forefront, and British self-interest at the top of the agenda, the Franco-German alliance would simply push ahead creating rules to suit their own interests.
There was a case for an all-party alliance to gain the public's support for a Yes vote in the referendum, he said.
In response to Tory leader Michael Howard's argument that only countries have constitutions, Mr Byers said the case for Europe was merely "a hard-headed recognition of how we can advance our national interests in the modern world".
The decision by Tony Blair to hold a referendum on the EU constitution raised some concern among other European leaders.
At celebrations to mark the accession of 10 new countries into the union in Dublin last weekend, the prime minister repeated his belief that holding a referendum was "the right thing to do".
"I hope very much we keep our place at the centre of Europe," he said.
"I think it is vital for our country. But in the end that is a decision that people have got to take."
Labour backbencher Ian Davidson, who set up the Labour Against a European
Superstate, said Mr Byers' patriotism argument proved there was no other case
for a 'Yes' vote. He branded it the "Dad's Army defence".
He said Britain had very little to gain from accepting the constitution. If it
said "no" it would remain part of the EU and retain all the attendant
He added that Mr Byers was undermining Britain's position by calling for a "Yes"
vote before the constitution had even been finalised.
That weakened Tony Blair's
negotiating hand, he said.