The government says it will reject an Italian draft of the new European Constitution because it would remove member states' veto on foreign policy.
Ten countries join the EU next year
A Foreign Office spokesman said the draft by Italy, which holds the EU presidency, is unacceptable although more discussions are due on Friday.
The new constitution is designed to ensure the EU works properly when it expands to 25 members next year.
But the Conservatives will step up their opposition to it on Thursday.
They are expected to use a foreign affairs debate in the Commons to accuse the government of misleading the public about the true nature of the constitution, which they claim reduces British sovereignty.
They will press ministers to spell out exactly where they stand, after a senior government source suggested this week there might not be a deal on the new constitution at all.
All 25 governments must agree on the text before the treaty can be put into effect.
The UK government has always said that control over foreign policy is one of its so-called "red line" issues, where it will not give up its veto.
A Foreign Office spokesman said the Italian draft would be rejected in its current form, but remained the basis for more discussions by EU foreign ministers in Naples on Friday.
On Tuesday, it emerged that Britain might not reach an agreement with its European Union partners about a new constitution.
A senior government source said that while the constitution was "highly desirable" it was not essential.