There is "every reason to believe" the UK will achieve the concessions it has demanded in the new EU treaty, the European Commission's president says.
EU leaders are due to sign the reform treaty later this year
Opt-outs over areas including foreign policy and tax had been "hard fought for", Jose Manuel Barroso added after meeting Gordon Brown in Downing Street.
Both the Conservatives and the UK Independence Party repeated calls for a referendum on the treaty.
Such demands have been consistently rejected by the British prime minister.
Mr Brown has threatened to veto the treaty if he does not win the concessions - or "red lines" - he has sought.
"I think that is well known to people across Britain," he told a news conference.
"I'm a cautious man and I will wait until we see the discussion which takes place in the council next week before I make a judgement on this," he said.
This was a reference to an informal summit in Lisbon where leaders will gather to iron out any remaining issues on the treaty.
"I have got to be absolutely sure that the British national interest has been safeguarded and that it is protected in every one of the issues that we have raised", he said.
Failure to achieve these "red lines" would mean "we could not accept the amending treaty," Mr Brown insisted.
Mr Barroso said at a press conference he was "well aware of the specific concerns of Britain" regarding the agreement.
But he added it was important for EU nations to "pull together" to continue to be "a Europe of results" which delivered "concrete benefits to the citizen".
And he said the UK was an "absolutely indispensable" part of the EU.
"I have every reason to believe the opt-outs that were so hard fought for by Britain are going to be kept in the text. Now we need to have this matter settled and move on."
Mr Barroso said the EU must take a "tougher stance" on Burma
However, the Tories' spokesman on Europe, Mark Francois, said: "Despite Brown's cynical spin in the run up to Lisbon, the 'red lines' are now collapsing under detailed scrutiny.
"Eighty per cent of the British public, the trade unions and MPs from all political parties are calling for the referendum that Labour promised them, and the British people must be allowed to have their say."
UKIP leader Nigel Farage suggested Mr Barroso was being "utterly dishonest" in suggesting the treaty could be amended in Britain's favour.
"That is not true and he knows it. The treaty is now set in stone," Mr Farage said.
"Brown knows that he won his seat promising a referendum. Barosso knows it, and what is more, every man, woman and child in our country knows it.
"Anything less than a vote would be a dereliction of duty".
If agreement is reached as expected, the way is clear for the treaty to be formally agreed by the leaders at a conference in December.
Under a timetable envisioned by Germany and backed by several member states, the treaty should be ratified by the national parliaments of all EU member states by mid-2009, ahead of the next European elections.
Mr Barroso was also asked about the prospect of being in the same room as Zimbabwe's president, Robert Mugabe, at an EU-Africa summit in December. Mr Brown is boycotting this event.
"Very often we have to sit in international meetings with people my mother would not like me to sit with," Mr Barroso joked.
"But I will make clear our position on human rights, and that we expect freedom of expression in Africa and all over the world. I will not miss the opportunity to do that."