European Commission President Jose Manuel Barroso has urged Britain not to hold a referendum on any new treaty agreed at this week's EU summit.
Mr Barroso said new EU members should help the community
He said it was up to the British people but he could not see why the country which exported democracy to the world would not respect its own Parliament.
And he urged all EU leaders to resist "ugly nationalism" in talks later.
The UK government, which opposes key parts of the treaty, is under pressure to hold a referendum on any deal.
The summit is meant to issue a mandate for an intergovernmental conference to agree the precise wording of a treaty to replace the failed EU constitution.
Arriving in Brussels for the summit, UK Prime Minister Tony Blair said he would not budge on his four "red lines" - foreign policy, tax and benefits, common law and a charter of fundamental rights, which could give workers more rights to strike.
"It is going to be a very tough negotiation," said Mr Blair.
"We have laid down four areas where we have to have really significant change and we will have to see that change - it will have to be done.
"On the other hand, of course, we want to make sure that Europe works more effectively. There are almost double the number of countries in Europe today, and we need to get this resolved, and I hope very much that it can be done."
UK Foreign Secretary Margaret Beckett has said Britain wants a Europe "of sovereign nations, not a superstate".
'Killing their king'
But Mr Barroso warned Britain not to block progress towards a treaty.
In an interview with the BBC, he said: "Sometimes I hear people saying that for Parliament to approve it would be by the back door.
"Britain is the country that exported Parliamentary democracy to the world. Do the British people consider Parliament the back door?
"Do the British people who killed their king to protect the rights of Parliament consider it the back door?
"Is that the respect some people show their Parliament, maybe the greatest Parliament in the world? I don't consider Parliament the back door."
He added that leaders had to stand up to the sort of "ugly nationalism" that traded on "imaginary threats" like the idea the EU was becoming a superstate.
He said he could not see why a confident country like Britain was not confident about Europe.
And he suggested Britain would leave the talks having ceded some ground.
"All the leaders say that they want 100% achievement," he said.
"But one thing we have to accept is that in Europe you've got to be reasonable and rational - and I know that Prime Minister Blair, as the British people, is very reasonable and rational."
It comes as an opinion poll suggests the majority of British people want a referendum on the new constitutional treaty and do not want to give more powers to the EU.
The ICM poll of 1,000 people, for eurosceptic think tank Open Europe, also suggests that refusing to hold a referendum could damage Labour at the next election.
Mr Blair, who promised a referendum on the EU constitution in 2004 before it was rejected by voters in France and Holland, said a vote would not be needed if Britain got its way at this week's summit.
But the Conservatives say any deal handing power to the EU must be put to the public vote.
Shadow Foreign Secretary William Hague said: "These red lines are so vague that we may have to debate afterwards whether they have been met or not.
"And even if he did meet all of them - even if he managed to defend all of them and could define what they were - he would still have given over a great deal more power from Britain to the European Union.
"And that's why we think he should keep his promise to have a referendum on this kind of treaty."