LONDON (AP) — Britain’s House of Lords launched a bid Monday to gain a greater say for Parliament over the terms of the U.K.’s European Union exit, as Prime Minister Theresa May warned lawmakers not to hold up “what the British people want” by trying to delay the process.

The Lords, Parliament’s unelected upper chamber, began two days of debate on a bill authorizing the government to trigger the EU divorce process.

The Lords can’t overrule the elected House of Commons, which has already passed the bill. But opposition members of the Lords are seeking amendments to guarantee Parliament a bigger say in the negotiations. Any amendments they pass would go back to the Commons for approval.

That parliamentary “ping pong” could delay the bill’s passage, endangering the government’s self-imposed March 31 deadline for triggering two years of Brexit negotiations.

Angela Smith, the opposition Labour Party’s leader in the Lords, promised that “we will not block, wreck or sabotage the legislation before us.”

But, she added, “neither should we provide the government with a blank check.”

Peers packed the red leather benches of the Lords chamber, which does not have enough seats for all 800 members of the house. In an unusual move for a prime minister, May sat inside the chamber to watch the opening of the debate – a move some saw as a warning to peers not to impede the will of the people.

Speaking earlier on a visit to the central England city of Stoke-on-Trent, May noted that the House of Commons approved the bill earlier this month without amendments.

“I hope that the House of Lords will pay attention to that,” she said.

Natalie Evans, the Conservative leader of the Lords, said Parliament must respect voters’ decision in a June referendum to leave the 28-nation bloc. She warned colleagues not to try to “restrict the government’s hand before it enters into complex negotiations or attempt to re-run the referendum.”

But Smith said the Lords would fulfill its role as the chamber of sober second thought and hold “a serious and a responsible debate.” Almost 200 peers are due to speak during the debate on Monday and Tuesday.

“If we ask the House of Commons to look again at an issue, it is not a constitutional outrage, but a constitutional responsibility,” Smith said.

Opposition amendments will seek to guarantee a vote for Parliament – or even a second referendum – on the deal agreed between Britain and the bloc.

Another change will seek to guarantee the rights of 3 million EU citizens living in the U.K. after Britain leaves the bloc. So far Britain and the EU have failed to ensure that the EU nationals – and 1 million Britons living in other parts of the bloc – can stay, despite repeated assertions by both sides that people shouldn’t be used as leverage.

Scores of EU citizens and their supporters rallied outside Parliament Monday to lobby lawmakers, carrying EU flags and stickers declaring: “I am not a bargaining chip.”

Former Foreign Secretary William Hague – now Lord Hague – said Parliament should pass the bill, let the government start negotiations and “end the uncertainty” surrounding Britain’s future.

“The country cannot go around in circles,” he told the House of Lords.