Theresa May is continuing to hold talks with European leaders and EU officials aimed at rescuing her Brexit deal.
No 10 said Mrs May’s meeting with Angela Merkel had been positive and the PM was determined to get MPs to back her deal.
Many of her MPs are demanding legally-binding guarantees over the backstop plan for the Irish border.
The German leader said the deal could not be re-negotiated but she was still optimistic a solution could be found.
But Labour leader Jeremy Corbyn labelled Mrs May the “runaway prime minister” and said the trip was a “waste of time and public money”.
The prime minister, now in Brussels, is seeking “further assurances” that the UK will not be trapped in the Northern Ireland backstop plan indefinitely.
Critics object to the backstop – a temporary customs arrangement designed to prevent the need for checkpoints at the Irish border if a long-term solution that avoids them cannot be agreed – because it imposes different regulations in Northern Ireland from the rest of the UK.
They also object to the fact that under the terms of the withdrawal agreement, the UK can not exit the backstop without the EU agreeing.
Irish Prime Minister Leo Varadkar, who will welcome Mrs May to Dublin on Wednesday, said he hoped to reassure the UK without changing the fundamental substance of the withdrawal deal, including the backstop.
“Our approach is that we have a deal on the table,” he told the Irish Parliament. “Our objective is to get the deal ratified by the House of Commons.”
What is Theresa May asking for?
Commons leader Andrea Leadsom said Mrs May was seeking to give the UK Parliament a vote on whether to enter the backstop – and an annual vote on whether the country should remain in it.
She told BBC Radio 4’s Today programme this could be done in an “addendum” to the withdrawal agreement, without changing the main text of it.
What next for the vote on deal?
MPs have to give the go-ahead for Mrs May’s deal if it is to come into effect when the UK leaves the EU on 29 March.
Downing Street has said a Commons vote will be held on the deal before 21 January and Brexit minister Robin Walker told MPs he hoped it “would be sooner than that”.
Mrs Leadsom earlier suggested talks with the EU could go right down to the wire, saying: “The EU is always in a position where it negotiates at the last possible moment.”