Britain's Prime Minister Theresa May addresses the House of Commons in London, March 14, 2018. Parliament TV handout via REUTERS

LONDON (Reuters) - Prime Minister Theresa May faces a June 12 showdown with potential Brexit rebels, when the legislation underpinning Britain’s exit from the European Union returns to the floor of parliament for a crucial debate.

The unelected upper house of parliament has infuriated the government by making 15 separate changes to the exit legislation despite ministers’ efforts to block them. Those changes include core Brexit issues such as whether Britain should leave the EU’s single market and customs union.

The government will ask lawmakers in its directly-elected lower house to overturn some of those changes during a debate expected to run late into the night.

The debate will test May’s ability to broker a compromise with those in her party who agree with the upper house and favour a less radical departure from the EU - including a contingent who want Britain to remain in the EU customs union.

Failure to placate those rebels could force a tight vote with serious consequences for both May’s authority as leader of a minority government and the future trading relationship between Britain and the EU.

A journalist for The Times newspaper tweeted a letter from the top official responsible for making sure lawmakers in the ruling Conservative Party vote in support of the government, asking them to make sure they will be working in parliament on June 12 and available to vote.