Britain's requests for concessions in the EU treaty are likely to be met, but it must not make any fresh demands, the European Commission's president says.
Mr Barroso said a "broadly agreed" EU treaty would be a good outcome
President Jose Manuel Barroso added he hoped for no further "difficulties" at a two-day summit in Lisbon.
Prime Minister Gordon Brown has now endorsed the treaty, saying Britain can set its own policies on justice, home and foreign affairs, plus security.
He again rejected a Tory warning that a referendum on the treaty was needed.
European Commission President Jose Manuel Barroso has urged all EU leaders to back the treaty.
"We prefer to have a solution that is broadly agreed with some specific opt-outs for some countries than not to move forward," he said.
"Britain has negotiated very hard some opt-outs on some specific issues - now we don't expect any more requests in that area.
"We should respect these lines that were put forward by Britain, so I think there will be no more difficulties. I hope that we will have a consensus today or tomorrow on the reform treaty."
The opt-outs, which Mr Brown refers to as "red lines", are in areas such as human rights, tax and benefits, foreign policy and justice.
The prime minister said that if these made the final draft of the treaty, it would avoid any significant transfer of power to Brussels.
Mr Brown said Britain's interests would be "protected" with opt-outs
"I've been determined that Britain will continue to decide in justice and home affairs - and I believe that the detailed changes that are being made with the opt-in for Britain in this area protect the British national interests," Mr Brown said at a news conference in the Portuguese capital.
"On foreign affairs and security matters, it is important for us that Britain can decide, and that's why we have been determined that foreign policy remains inter-governmental and decisions are made by unanimity.
"On social security, we have been determined that there is an emergency break - and in some cases a veto - so that decisions are made in the interests of Britain."
Mr Brown, attending his first EU summit as prime minister, has pledged to veto the treaty if Britain's "red lines" were not fully incorporated.
But shadow foreign secretary William Hague said that by failing to agree to a referendum, Mr Brown was "still treating the British people like fools" with comments that had "reached new depths of cynicism".
"He still claims that because the name 'constitution' has been dropped, this treaty is somehow different, even though the European Scrutiny Committee has specifically told him his argument is misleading.
"He claims that this treaty is about making a free-trading Europe work better, when he knows that it downgrades the importance of free competition."
Mr Hague said polls showed most British people wanted a referendum
Mr Hague, a former Conservative leader, asked why voters should trust Mr Brown "when he so clearly does not trust them".
The UK Independence Party has also demanded a referendum, along with some Labour MPs, while ex-Lib Dem leader Sir Menzies Campbell has said a public vote should be held on the wider question of UK membership of the EU as well.
But Mr Brown was adamant this was not needed.
"If we were debating as big an issue as Britain's membership of the euro, I would have been the first - indeed, I was the first - to say this is such an issue of great significance that the British people must vote in a referendum," he said.
"If it was the previous constitutional treaty, I would have argued, as we did, that there should have been a referendum. But this is an amending treaty, where the constitutional concept has been abandoned."
He said this was "a very different document" to the failed EU constitution, on which voters in the UK were promised a referendum.
And a parliamentary debate would be "the proper way of discussing this", he insisted, as long as the "red lines" made the final draft.