European Commission President Jose Manuel Barroso has told EU members not to block progress towards an EU treaty.
Mr Barroso said new EU members should help the community
Mr Barroso warned against taking a tough line into negotiations, telling member states that opposition to the treaty was not in their interest.
The UK and Poland are opposed to key elements of the treaty, with Warsaw against changes to voting rules.
France and Spain have proposed a 10-point document backing an expansion of EU powers.
Germany, which holds the rotating EU presidency, has aimed to put core elements of the aborted 2005 EU constitution at heart of a new agreement.
However, British and Polish objections have dominated the build-up to two days of negotiations at the end of this week.
Mr Barroso did not refer to the UK or Poland directly, but refused to deny that their opposition was causing problems.
"It is not in the interest of any member state to be in a position that is seen as hardliner," he said in Brussels.
"The environment for a deal is clearly there. Please avoid appearing as blocking. This is not intelligent, this is not in your interest.
"Defend your positions, but don't come with these red lines and vetoes."
UK Prime Minister Tony Blair has listed four so-called "red lines" that the British government insists it will not cross during negotiations.
They are: a Franco-Spanish call for a Charter of Fundamental Rights; and EU-wide legislation on foreign policy, common law and tax and benefits.
Mr Blair's successor-in-waiting, Chancellor of the Exchequer Gordon Brown, said on Tuesday that he was confident the UK would get what it was seeking from any treaty.
Polish Prime Minister Jaroslaw Kaczynski said Warsaw's pursuit of a greater share of voting rights was not negotiable, adding: "There is no plan B."
But Mr Barroso issued a coded warning to Poland, a new EU member, saying newcomers had a responsibility to help bridge divides within the union.
"It will be in their interest for them to show that their membership of the European Union is not making European Union life more difficult but, on the contrary, they are giving more impetus to the European Union," he said.
Polish negotiator Marek Cichocki said that his country was open to discuss various formulas to achieve a better balance between the big states and the others, but would not back down, said the BBC's Oana Lungescu from Brussels.