David Cameron's campaign to lead the Conservatives suffered a setback last night after Tory MEPs warned his eurosceptic stance could split the party.
West Midland MEP Philip Bushill-Matthews said: "He has not only shown bad judgment: he has also chosen to listen to some very bad advice."
The criticism followed an announcement by ten eurosceptic MPs that they were now backing Mr Cameron.
The MPs, including Bill Cash (Stone) and Daniel Kawczynski (Shrewsbury & Atcham), had previously supported Liam Fox, the Shadow Foreign Secretary, who has already been knocked out of the leadership contest.
In a joint letter, they said: "He has huge talent, intellectual ability and sound judgment and we believe he can appeal across the political spectrum to voters who did not support us in the past three general elections."
Mr Bushill-Matthews criticised Mr Cameron over his pledge to end the Conservative Party's alliance with the European People's Party in the European Parliament.
The EPP is a grouping of Christian Democrat parties, the leading centre-right party in most EU countries.
Unlike the Tories, Christian Democrats have traditionally supported a federal Europe.
But Mr Cameron has wooed Eurosceptics by saying Tory MEPs should pull out of the EPP.
Mr Bushill-Matthews said the Conservatives had made a manifesto commitment to remain part of the EPP.
He said: "I urge David Cameron not to encourage colleagues to break such a clear pledge, not to weaken our ability to deliver our manifesto commitments, and not to create new splits over Europe when he should be uniting our party to replace the present Government."
A spokesman for the Cameron campaign said: "He thinks it simply isn't sensible to be part of a group which takes a view of the EU which is completely contrary to the Conservative Party view."
This will be a crucial week for both candidates. They are set to appear together in a televised debate on BBC 1's Question Time on Thursday, and ballot papers will be sent out on Friday.
The leadership contest will come to the Midlands again in two weeks, when hustings are held in Birmingham.
The two candidates are to take part in an event for party members in the city on November 14. Similar events will then be held in Leicester, Newcastle, York and Bolton.
Mr Davis launched the national stage of his election campaign in Warwick, while Mr Cameron launched his in Birmingham.
Last night Mr Cameron placed the environment at the heart of his campaign when he spoke in Cornwall.
He called for the creation of a new watchdog to monitor carbon emissions and provide an independent assessment of Government efforts to stop global warming.
Mr Davis recently promised to cut taxes by £1,200 for the average family and to end Labour's tax grab on pensions.
Mr Cameron is seen as the clear favourite, and the strength of his lead was illustrated by a BBC survey which showed 76 per cent of party members who had already chosen a candidate were backing him.