Britain's Prime Minister Theresa May speaks at the formal opening of the Commonwealth Heads of Government Meeting in the ballroom at Buckingham Palace in London, Britain, April 19, 2018. Yui Mok/Pool via Reuters

LONDON (Reuters) - Britain’s lower house of parliament will debate staying in a customs union with the European Union next week, stepping up pressure on Prime Minister Theresa May to change her Brexit blueprint.

Opposition Labour and pro-EU Conservative lawmakers have backed having a so-called backbench debate next Thursday after the upper house dealt a blow to May by challenging her decision to rule out staying in the bloc’s customs union.

The debate, which if put to a vote will be largely symbolic and will not instruct the government to follow its conclusions, adds pressure on May, who has stuck to her argument that only by ending Britain’s membership of the union will London be able to strike new trade deals independently of the EU.

The debate, backed by Labour’s Yvette Cooper who chairs parliament’s home affairs committee, “calls on the government to include as an objective in negotiations on the future relationship between the United Kingdom and the European Union the establishment of an effective customs union between the two territories”.