LONDON, March 6 (Reuters) - British opposition leader David Cameron will accuse the government on Tuesday of weakness in dealing with its European partners and pledge he would be "solid and consistent" in defending British interests if he won power.

Cameron, who has reinvigorated his traditionally eurosceptic Conservative Party after it lost three elections in a row, will say in a speech in Brussels that British politicians sometimes posture for television back home but then agree to whatever EU proposal is put before them.

He will cite as an example the way Labour Prime Minister Tony Blair and Finance Minister Gordon Brown conducted negotiations on the European Constitution, calling it a study in how not to deal with the EU.

"At the beginning of the process, Tony Blair and Gordon Brown ... spoke of the 'red lines' which the government would not, under any circumstances, cross. But in the event, they gave their total assent to the text," he will say, according to excerpts of the speech released in advance by his office.

The EU Constitution has been in limbo since it was rejected by French and Dutch voters in referendums in 2005, leading Blair to suspend plans for a British referendum. The treaty is meant to streamline decision-making in the 27-nation bloc.

German Chancellor Angela Merkel, whose country holds the EU's six-month presidency, is trying to resuscitate the treaty.

Cameron said he was against a European Constitution and in favour of a referendum.

"I believe that the best way to pursue your national interest is not to posture, but to persuade. I will be polite, but solid and consistent," Cameron will say, urging the EU to concentrate on globalisation, climate change and global poverty.

Cameron's Conservatives consistently lead Labour in opinion polls. Blair has pledged to step down this year after a decade in office but a general election is not expected before 2009.

Cameron will be speaking at the first conference of the centre-right Movement for European Reform, which the Conservatives say is open to European parties which share their belief that the EU needs to change direction.

The Conservatives and the right-wing Czech Civic Democrats agreed last July to break away from the European People's Party, the European Parliament's biggest political grouping.

They will launch a new alliance in the parliament to push for EU reform -- but not until 2009.

Conservatives say the EPP, made up of Christian Democratic and conservative lawmakers, supports a federal Europe which is at odds with the British party's more eurosceptic stance.

Cameron said last July he was confident many other parties would join the new alliance, but the Labour Party poured scorn on it, saying it would leave the Conservatives isolated.

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