Tensions between Gordon Brown and Europe’s top diplomat were laid bare yesterday as the Prime Minister tried to kill off the case for a referendum on the EU’s reform treaty.

After meeting José Manuel Barroso, the European Commission President, Mr Brown expressed cautious optimism that Britain’s demands had been heeded and that he could approve the treaty at an informal EU summit in Lisbon next week.

“I am a cautious man and I will wait to see the discussion that takes place in the [European] Council next week before I make a final judgment,” Mr Brown said. “But if we achieve our red lines and achieve them in the detail then we would not need to veto the treaty, we would not need to come back and say it was unacceptable.”

The Commons European Scrutiny Committee, which this week expressed doubts about the value of Britain’s so-called red lines, released a letter saying the treaty’s new text contained extra provisions that threatened to undermine the right secured by Britain to opt in to EU criminal justice initiatives.

Mr Brown’s remarks were intended to end confusion after a press conference on Monday at which he suggested he could veto the treaty or hold a referendum if Britain’s red-line negotiating conditions were not respected.

But Mr Barroso made clear his frustration at the stance, saying Britain should not be “closed to Europe” and openly disagreeing with Mr Brown’s decision to boycott a summit of EU and African leaders in December to be attended by President Mugabe of Zimbabwe..

The two men appeared side by side for a joint press conference after their meeting in Downing Street.

Several times Mr Brown referred to having “made clear” his position to Mr Barroso on the reform treaty. Only once did he praise his guest, congratulating Mr Barroso on Europe’s leadership in tackling climate change.

Mr Barroso responded by suggesting Britain’s uncompromising tone on the treaty talks was unnecessary, saying he had “every reason” to believe the opt-outs and other conditions would be honoured in the treaty.

The Commission President urged Mr Brown to play a more positive role in the EU, saying: “I think it is absolutely impossible, and I cannot believe, that Britain can be open to the world and closed to Europe. We really need British engagement on this.”

Last night, Mr Barroso explained his frustration further, saying in a lecture at St Antony’s College, Oxford, that he found it “frankly strange” that Britain was often portrayed as being fundamentally at odds with Europe.

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