Cameron is seeking to renegotiate ties with the European Union before a referendum. Divisions over Europe contributed to the downfall of both Margaret Thatcher and John Major, the last two Conservative prime ministers before Cameron.
Out of 330 lawmakers, 67 are either 'firmly out' or 'out leaning' while 203 could vote either way, according to research by Open Europe based on public statements and voting on EU issues, Reuters reports.
Just 14 lawmakers were firmly for staying in the EU and 44 were leaning towards staying in, Open Europe said. 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.
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."