LONDON (Reuters) - British Prime Minister Boris Johnson said on Saturday that "tremendous progress" was being made to reach an agreement on Brexit in an interview published in the Mail on Sunday newspaper.

The upbeat comments from Johnson coincide with the split of another lawmaker from his party who chose to join an opposition party opposed to Brexit.

"When I took over this job, everyone was saying that there could never be any amendment to the withdrawal agreement," Johnson said. "They have retreated on this and, as you know, there is a very good dialogue on how to address the problems of Northern Ireland's borders."

Negotiations are stalled over how to avoid the return of the physical border between the EU-member Republic of Ireland and the British Northern Ireland province after Brexit.

London rejects the "safety net" clause, which states that if a solution is not reached at the end of a transitional phase, the entire United Kingdom remains in a "unified customs zone" with the EU, with the aim of preventing the return of a physical border between Northern Ireland and the Republic of Ireland.

But Johnson believes that this provision will prevent the United Kingdom from adopting an independent trade policy, as it will remain subject to European laws.

Johnson is scheduled to meet in Luxembourg tomorrow with European Commission President Jean-Claude Juncker and EU negotiator in charge of Brexit Michel Barnier.

Johnson, meanwhile, suffered another setback on Saturday as another lawmaker from his own party opted to join an opposition party that was reluctant to break.

Former university minister Sam Gemma joined the LDP at its annual conference in Bournemouth, southern England.

Gemma regretted that "Boris Johnson offers a difficult choice for moderate and progressive lawmakers in the Conservative Party: accepting Brexit without agreement, or leaving public life."

"I choose to continue the fight for the values ​​I have always believed in as a liberal democrat."

Gemma's split comes a few days after conservative Rep. Philip Lee, who lost his majority in parliament.

Gemma is the sixth lawmaker this year to join the Liberal Democratic Party, which now holds 18 of the 650 seats in the House of Commons.