British voters have been warned they could open up questions about the UK's relations with other European countries if they reject the EU constitution.
Britain holds the EU presidency for six months in 2005
The EU's foreign policy chief, Javier Solana, told BBC Radio 4's Today that the constitution was important for making the EU more efficient.
He said he was sure Prime Minister Tony Blair would mount a solid defence of the constitution ahead of the vote.
The referendum, which Tories say is flawed, is expected in spring 2006.
Mr Solana said a No vote in the referendum would not exclude Britain from the "European family".
But questioned about the implications of a No vote, he said: "It will open up the question of the relationship with other members of the EU that have voted yes."
Mr Solana said the constitution marked a "fundamental step" in making the EU better able to work internally and fulfil its international obligations.
Each of the EU's 25 member states must approve the constitution by parliamentary vote or referendum before it can take effect.
So far, Hungary and Lithuania - both new members - have ratified the constitution with a parliamentary vote.
The first popular referendum is expected to take place in Spain in February.
The constitution - agreed in 2004 - intends to make the union function more smoothly and includes an increase in the policy areas where countries lose their national veto.
It will also create a foreign minister's post.
Labour says the constitution is necessary to speed up decision making in an enlarged EU but the Tories argue it would be bad for Britain.
In November, Michael Howard said: "The constitution will make Europe's economy even less flexible, even less competitive and even more sluggish than it is today."
Mr Blair denied the Tory claims and argued that progress had been made on making the EU economy more competitive.