"If I were to come to the conclusion that we were not having the detail of what was decided reflected in the final outcome of the declaration then of course I would come back to the British people and say, look, we have to do things differently."

The London threat comes as EU foreign ministers gather at the end of the week for the first political discussion on the treaty since its broad outline was agreed by EU leaders in June.

The June agreement, reached after hours of bitter negotiation, saw London secure key opt-outs from the treaty, including from a citizens rights charter and justice and home affairs policy.

Now that the political dust has settled, technical and legal experts are wondering at the full implications of these opt-outs and how it will affect other member states down the line.

However, Mr Brown cannot be seen to soften on any of these issues. He is being closely watched by opposition conservatives and members of his own labour party who are demanding a referendum, arguing that the new treaty is very similar to the original EU constitution.

The government had previously said it would have a referendum on the constitution.

Mr Brown's case rests on saying that the opt-outs secured by Britain make the treaty sufficiently different.

In the coming days, he will face two big challenges to this position. On Monday (10 September) trade unions will meet for an annual congress. They are pushing for a poll, angry at Britain's opt out from the rights charter, which among other things, secures the right to strike.

Meanwhile, the Labour conference on 23 September will provide a platform for Labour rebels to make their case for a referendum.

Not just the British

But Britain is not alone in causing the EU presidency Portugal - which wants a quick and painless agreement on the treaty - some headaches.

Poland, a similarly tough negotiator, plans to have early elections in October. This may also affect Lisbon's plans to get the whole treaty politically agreed at a meeting of EU leaders next month.

Polish daily Rzeczpospolita reports that Portuguese diplomats are wondering whether to delay the summit because of the elections.

However, a Portuguese spokesman flatly denied the rumour. "I can tell you 'no'. No, that is not true," he told EUobserver, adding that it was something that was not even being considered.