The former prime minister's intervention came as the UK government admitted a new bill on post-Brexit Northern Ireland arrangements contravene international law.
Boris Johnson's government has been taken to task by the prime minister's predecessor Theresa May over reports that the UK may renege on binding post-Brexit arrangements for Northern Ireland.
Her intervention came in the House of Commons on Tuesday afternoon, where the minister for Northern Ireland admitted that a planned new bill "does break international law".
The former occupant of `10 Downing Street reminded MPs that the UK government had signed the Withdrawal Agreement -- part of the divorce deal that took the country out of the EU -- with the Northern Ireland Protocol, and the British parliament had voted the deal into UK law.
"The government is now changing the operation of that agreement. Given that, how can the government reassure future international partners that the UK can be trusted to abide by the legal obligations of the agreements it signs?" she asked.
The Northern Ireland Secretary replied that the British government had worked "in a spirit of good faith" with the EU "to ensure that we do implement the arrangements that uphold the fundamental principles that lie behind the Protocol".
"But the Withdrawal Agreement and Protocol are not like any other treaty. It was written on the assumption that subsequent agreements could be reached between us and the EU on the detail," Brandon Lewis said, adding that the government had a duty to provide certainty for businesses in Northern Ireland.
UK admits bill 'breaks international law'
The minister confirmed that draft legislation on the UK's future arrangements for Northern Ireland -- due to be published on Wednesday -- "does break international law, in a very specific and limited way... in certain, very tightly defined circumstances".
Lewis said there were precedents where the UK and other countries needed "to consider their international obligations as circumstances change".
On Monday European Commission President Ursula von der Leyen warned the United Kingdom that any attempt to row back on its Brexit commitments could jeopardise a future trade agreement and put at risk hard-won peace on the island of Ireland.
The Financial Times reported earlier that the government was planning to override its divorce deal obligations concerning Northern Ireland with new UK law. Ministers and officials later played down its significance, saying the government remained committed to the agreement and wanted to to tie up "loose ends".
Meanwhile the head of the government's legal department, Jonathan Jones, has resigned, reportedly over concerns that the UK may renege on parts of the divorce deal.
This came on the day that UK-EU talks on trade and the future relationship resumed in London. Boris Johnson has threatened to pull the UK out of negotiations unless a deal is struck by mid-October, ahead of the expiry of the post-Brexit transition period on December 31.
Northern Ireland conundrum returns
The Withdrawal Agreement is the legally binding part of the divorce deal that Boris Johnson's government struck with the EU in the autumn of 2019, paving the way for the UK finally to leave the bloc in January this year. Setting out the terms of the UK's departure, it has the force of an international treaty.
Arrangements concerning the politically sensitive border between the UK territory of Northern Ireland and the Republic of Ireland, an EU member, were at the heart of the British parliamentary deadlock that led to Brexit being delayed in 2019, and contributed to Theresa May's downfall.
Her ill-fated plan contained the infamous "backstop" measure: this would have kept the whole of the UK inside the EU's Customs Union as a backup solution to keep the border open, should no UK-EU trade deal be agreed.
After May was replaced by Boris Johnson, the new leader ditched the plan and struck a new deal with the EU. The altered agreement also guarantees an open border on the island of Ireland -- but to accommodate the UK's avowed wish to diverge from EU rules, instead creates complex arrangements between Northern Ireland and Britain.
Under the revised Protocol, although Northern Ireland will leave the EU's Customs Union after the end of the transition period, in practice it will follow EU customs rules and be subject to EU oversight. It will also remain partially aligned to EU single market standards.
This effectively creates a new regulatory divide in the Irish Sea between Great Britain and Northern Ireland, implying some checks on goods. But the British government's insistence that trade will continue to be frictionless, and criticism of the state of preparations for new infrastructure, have called into question its commitment to the treaty.
The British government has said it is working “in good faith” to implement the agreed border provisions but is “considering fallback options in the event this is not achieved". In the Commons, Lewis repeatedly stressed the part of the Protocol which guarantees that Northern Ireland will have "unfettered access" to the UK's internal market.