Britain’s former ambassador to Washington has described the internal market bill as “hugely damaging to our international reputation”, warning that it could deter other countries from entering into agreements with the UK in the future. Kim Darroch told the Guardian that if it becomes law, in violation of the withdrawal agreement signed with the EU last year, it would endanger the country’s prospects of achieving trade deals with the EU and the US, and have an enduring impact on its global standing. “It is the most astonishing thing I can recall through my entire public service career: a minister said we are knowingly breaking international law,” said Darroch in an interview. Over the course of a 43-year career at the foreign office, he served as envoy to the EU in Brussels and national security adviser, before becoming ambassador to the US. “It’s potentially hugely damaging to our international reputation. It puts at risk future international agreements, if people think the Brits are just going to say: we didn’t like this on reflection, and we would like to rewrite this part unilaterally,” the retired diplomat, now Lord Darroch of Kew, said. The bill survived its latest parliamentary hurdle on Monday night, passing its second reading in the House of Commons by a margin of 77 votes. Governing the flow of goods and services between the UK’s four constituent nations, the bill stipulates it takes precedence where it contradicts elements of the withdrawal agreement, and the British government has admitted that would break international law. Darroch said the impact could be first felt in negotiations with the EU to establish new trade relations after Brexit. “I’ve talked to a few people in Brussels, and they are determined not to just walk away from the table because of this. It’s equally inconceivable you would get the UK-EU deal through if this passes into law,” he said. The effect of the bill on Northern Ireland would have other ramifications, particularly if it led to customs checks on the border with the Republic of Ireland, something the withdrawal agreement was designed to prevent. The return of a hard border would jeopardise the 1998 Good Friday agreement, to which the US is guarantor. “The UK-US deal is at risk because, even if Donald Trump is reelected, it’s quite hard to see the Republicans taking back the House [of Representatives],” Darroch said. “And as long as the House is in Democratic hands, the leadership have made it clear that a UK-US free trade deal wouldn’t be possible if we put at risk the Good Friday agreement.” Darroch was forced to resign as ambassador to Washington in July last year after his reports back to London on the turmoil and dysfunction in the Trump administration were leaked. Donald Trump declared his officials would no longer deal with the UK ambassador, and Boris Johnson, who was at the time running to replace Theresa May as prime minister, failed to offer Darroch a clear statement of support. The former diplomat gives an account of his turbulent time in Washington in a memoir, Collateral Damage, published in the UK on Tuesday. Darroch speculated the internal market bill could be a negotiating ploy to force concessions from Brussels. If so, he said, it was not a very good one. “If we were to back down or find a way through with the EU, that would limit the damage, but the fact that we contemplated it is still damaging,” he said. “The downsides, with the EU free trade deal, the US free trade deal and our international reputation are so big, I can’t quite believe they will carry through on it.”