Gordon Brown, bruised after two weeks in which David Cameron's Tories came off the back foot with a captivating conference and followed through with parliamentary panache, faces an even tougher week ahead. A weekend poll giving the Conservatives a 7% lead over Labour and their best showing since before Black Wednesday merely illustrates what we already knew: that the floating voters of the English marginals want to be reassured that their expensive homes are as impregnable against the tax collector as any castle.

This week was always going to be the test. Even when the Prime Minister was at the top of the bounce he gained from standing firm against flood and pestilence - and from not being Tony Blair - he knew that the EU Treaty was lying in wait. On Thursday he goes to Lisbon to the EU summit on the European Reform Treaty to argue the case for his "red lines": the areas he regards as non-negotiable to protect Britain's interests. So far he has been scrupulously careful not to give any ground, while seeking to reassure our European partners that he supports the goal of reform of the European Union to better accommodate a membership of 27 and the new world order. All his pronouncements must play equally to the electorate at home, which fears ceding more control to Brussels, and to Britain's critics in the EU, who argue that individual opt-outs weaken the strategy against terrorism. But at the centre of this diplomatic minefield lies the potential nuclear bomb of a referendum on the issue.

In their manifestos for the 2005 General Election, both Labour and the Conservatives promised to hold a referendum on what was then the new EU constitution. Labour's position that the amended treaty falls short of a constitution and does not require a vote has been seized on by the Conservatives as a breach of promise. Yesterday his position became more precarious as the expected Tory jibes were overshadowed by a damning warning from Lord Falconer, the former Lord Chancellor, that the Labour Party must set out a clear vision or risk offering "drift, not leadership, and the past, not the future". Hints of further public criticism to come from high-profile former Labour ministers only holding fire until the current storm has abated deepen the split.

Following his vacillation over calling an election, Mr Brown now risks the charge of failing to honour the manifesto by which he holds office. Stating in simple terms what the opt-outs mean should be the first step towards regaining the public trust. And if the term "red lines" should happen to stir a remembrance of "the thin red line" of the Argyll and Sutherland Highlanders' unlikely routing of the Russian Cavalry at Balaklava, it must be remembered that it was achieved not only by extraordinary courage, but absolute trust in their leader.