Leading article: Mr Cameron's first big test as leader

David Cameron's honeymoon with his party is showing signs of strain. Tory right-wingers are beginning to criticise their leader's domestic pronouncements, while he is also set to be engulfed in his first internal party row over Europe. The shadow Foreign Secretary, William Hague, will be in Brussels this week to implement Mr Cameron's pledge to withdraw the Conservative Party's MEPs from the European People's Party. The shop steward of the Tory MEPs warned last weekend of "trench warfare" if this is enforced.

This policy of withdrawal is difficult to explain. Mr Cameron is making a show of moving his party to the centre ground at home, so why make a lurch to the right abroad that was ruled out by the likes of Michael Howard and Iain Duncan Smith? It will force Tory representatives in Europe to sit with extremists, fascists and Robert Kilroy-Silk. This does not fit comfortably with Mr Cameron's message that the Tories have learned from past mistakes.

Mr Cameron would do well to listen to his MEPs, the majority of whom want to stay in the EPP. Mr Hague spoke at the weekend of establishing a "new grouping" in Europe which would be less federalist than the EPP. But there is precious little evidence that any other centre-right European parties are about to follow the Tories. There was also brave talk from Mr Hague of forging a Europe able to meet the rising challenge from China and India. But he was vague about how the Tories' opting out of the EPP would facilitate this.

The reality is that the Tories would lose virtually all political influence in Brussels if they go through with this absurd plan. Their voice in the European Parliament - and that of Britain as a whole - would carry less weight, and their prominent role in the powerful committee structure would vanish. Their ability to modify legislation would be diminished, throwing up awkward questions. For example, how would Mr Cameron square this with his commitment to the environment? Most of our environmental legislation originates in Europe; indeed, a Tory is the EPP spokesman on the Environment Committee. So at the very time the Tories are claiming to be a green party, they are reducing their influence to force through measures to protect the environment.

Mr Cameron has a dilemma. Pulling out of the EPP was his one firm policy commitment when he ran for leader. This is, therefore, the first real test of his leadership. He must decide whether to provoke a long-running row with his own MEPs or to risk more flak from the right for reneging on an election pledge. In our opinion, he should find a way to wriggle out of his foolish commitment, despite the short-term damage to his credibility.