By Guy Faulconbridge
LONDON (Reuters) - As many as one in five of British Prime Minister David Cameron's lawmakers is likely to vote to leave the European Union in a referendum, new research showed on Friday, indicating the scale of the divide over Europe in the ruling Conservative Party.
For at least a generation, Cameron's party has been riven by a conflict over Europe that contributed to the downfall of both Margaret Thatcher and John Major, the last two Conservative prime ministers.
Under pressure from lawmakers who feared the electoral success of the anti-EU UK Independence Party, Cameron promised in 2013 a referendum on membership by the end of 2017 after renegotiating aspects of the EU relationship.
But as Cameron's party prepares to gather for its annual party conference, research from the London-based Open Europe think tank laid bare the scale of the divisions: Out of 330 lawmakers, 67 are either 'firmly out' or 'out leaning' while 203 could vote either way.
The research, based on public statements and voting on EU issues, showed just 14 lawmakers were firmly for staying in the EU and 44 were leaning towards staying in.
Pawel Swidlicki, an analyst at Open Europe, said the number of undecided lawmakers in the party showed "how important it will be for Cameron to secure a comprehensive and ambitious reform package in order to ensure that the bulk of his party, as well as the wider public, sees any recommendation to remain within the EU as credible."
Though the research is necessarily only an assessment, it is one of the first publicly available indications of the views of Cameron's lawmakers.
The Cabinet has 12 members who could vote either way out of 21 members with five leaning towards out and four leaning towards staying in, Open Europe said.
(Editing by Paul Sandle)