Shadow foreign secretary William Hague is to challenge Gordon Brown to "restore trust in British politics" by holding a referendum on the European Union's Lisbon Treaty.
Mr Hague will leave no doubt that opposition to the Treaty - which replaces the failed EU Constitution - will form the centrepiece of the Conservative campaign for the European Parliament elections on June 4.
And he will put the issue of "trust" at the heart of the campaign, arguing that Mr Brown's refusal to give the electorate a say on the Treaty amounts to a betrayal of Labour's manifesto promise of a referendum on the Constitution.
Labour has "debased the coinage of politics itself" and its legacy will be "to leave office with the word of government less believed than at any time in our lifetimes", he will say.
Speaking at the Conservative Spring Forum in Cheltenham, Mr Hague will say that it is not too late for voters to put pressure on the Government to give them a vote on Lisbon by backing the Tories in the European poll. And he will repeat the Conservative promise to hold a referendum on the Treaty if it remains unratified by any of the EU's 27 states when they win power.
Britain became one of 25 EU nations to ratify the Treaty after a parliamentary vote in June last year. The document is awaiting approval by the Czech Senate and President and must also overcome the hurdle of a second referendum in Ireland - where it was rejected in 2008 in the only national poll to be held anywhere in Europe - before it can come into effect.
Ministers deny breaching Labour's 2005 manifesto promise, arguing the Treaty is a different document from the Constitution, without the same constitutional significance.
Although the Tories insist that Lisbon is effectively the Constitution by another name, it is unclear whether they would seek to withdraw from the treaty if it is ratified by the whole EU before they reach office. Leader David Cameron has said only that he would not be happy to "let matters rest there".
Arguing that the Euro-elections will be "a campaign about trust", Mr Hague will say: "It is the saddest of all truths about this Government that faced with disillusionment with politics they have only added to it, faced with mistrust they have only justified it, and beset with cynicism they have opted only to exhibit it.
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"They have not only devalued the currency of the nation, but their breaking of promises has been so brazen, and in the case of the referendum so inexcusable, that they have debased the coinage of politics itself. Their legacy will be to leave office with the word of government less believed than at any time in our lifetimes - another aspect of the scorched earth they will leave behind them, on which only a new government can plant the seeds of trust and belief afresh."
