FILE PHOTO: Union Flags and European Union flags fly near the Elizabeth Tower, housing the Big Ben bell in Parliament Square in central London, Britain September 9, 2017. REUTERS/Tolga Akmen/File Photo

EDINBURGH (Reuters) - Nine members of Britain’s Conservative party are backing amendments to the legislation needed for Britain to leave the European Union, including a parliamentary vote on staying in a customs union, newspaper The Guardian reported on Friday.

The group of dissenters in Prime Minister Theresa May’s party includes two former cabinet ministers who are backing “substantial” cross-party amendments to the European Union (Withdrawal) Bill, the report said.

The changes drawn up include one giving parliament a vote on staying in a customs union “which is almost certain to pass”, the Guardian said.

The withdrawal bill is testing May’s ability to rule as her Conservatives are split on how best to enact Brexit, and she is dependent on a small party in Northern Ireland for her slim majority.

May has said that agreeing a new customs union, in which the EU’s 28 members operate as a single tariff-free market, with the EU would be a “betrayal” of the 2016 referendum result.

May’s working majority in the 650-seat house stands at around 13 seats. The group of Conservatives include Chris Patten, a former party chairman, and Douglas Hogg, a former agriculture minister.