"It's about authorising Parliament," she said. "It's to put wheels on the outcome of the referendum."
May's defeat means that she'll either have to accept the changes - which she's argued would weaken her in negotiations - or try to delete the amendments. Her team has signalled she's likely to try to overturn the Lords defeats when the bill returns to the House of Commons for further consideration, probably next week.
The PM wants to open Brexit talks by the end of this month by formally invoking Article 50 of the bloc's Lisbon Treaty. May hasn't named a date for making the move and there are conflicting reports on when she might pull the trigger, ranging from March 15 to March 31.
Today's amendment guarantees Parliament a veto over the final Brexit agreement, as well as the power to stop May walking away from talks without a deal, something she's threatened to do if she doesn't like the EU's offer.
May wants the 137-word draft to go through Parliament without changes to give her the maximum freedom to negotiate with EU leaders. Her spokesman, James Slack, told reporters that the PM is "absolutely" on course to trigger Brexit negotiations by the end of March.
Brexit Minister George Bridges told the Lords that while the Government has already promised them a vote on the eventual Brexit deal, enshrining that promise in the Brexit bill "could create a lucrative field day for lawyers" and hamper the UK.
"This amendment simply makes the negotiations much harder from day one for the prime minister," Bridges said. "It increases the incentive for the European Union to offer nothing but a bad deal."
May's office confirmed that the vote she's planning to give Parliament without the amendment would simply be one on whether to accept the deal or to leave without a deal.
- Bloomberg