Eurosceptic MP and former minister NIck Herbert who campaigned to keep Britain out of the euro will lead a new Tory group lobbying for the UK to remain part of the European Union, warning a Brexit would be a "jump into a void".
The new group, Conservatives for Reform in Europe (CRE), believes that the UK should remain part of a reformed EU.
"Leaving without the first idea of what we might get instead would be to jump into a void,” Herbert warned, writing in the Telegraph.
"It’s not hard to be vexed by some of the ways the EU works. But irritation is not enough to end a 40-year relationship. We need soberly to weigh up the costs and the benefits," he added, saying that some members of the group would however be prepared to leave new terms of membership were not agreed.
Conservatives are expected to begin aligning themselves behind either the pro-Europe or leave campaigns after Prime Minister David Cameron said ministers can make their own choice on which side of the debate they stand.
Education Nicky Morgan is the latest to reveal her position, arguing an exit would cut the UK off from the rest of the world and limit young people's prospects, writing in the Observer.
Commons leader Chris Grayling spoke out last week warning the UK's membership of the EU is disastrous for Britain.