There is no chance of Congress approving a US-UK trade agreement if Brexit undermines the Good Friday peace agreement in Northern Ireland, the speaker of the House of Representatives, Nancy Pelosi, has said.

Pelosi was restating the entrenched position of congressional Democrats and many Republicans in the wake of remarks made by Donald Trump’s national security adviser, John Bolton, during a visit to London this week.

Bolton had said that Britain and the US could sign interim, partial free trade deals, one sector at a time, which would go through the a fast track legislative process, to help the UK cope economically if there is a no-deal Brexit on 31 October.

In a statement on Wednesday, the House speaker, who commands a Democratic majority, warned that the Trump administration would not be able to sidestep congressional approval.

“Whatever form it takes, Brexit cannot be allowed to imperil the Good Friday agreement, including the seamless border between the Irish Republic and Northern Ireland, especially now, as the first generation born into the hope of Good Friday 21 years ago comes into adulthood.” Pelosi said. “We cannot go back.”

Bolton said the sectoral deals, focusing on industries such as car manufacturing, could be negotiated quickly, and insisted they would receive overwhelming bipartisan support in Congress.

“The ultimate end result is a comprehensive trade agreement covering all trading goods and services,” he said after meeting Boris Johnson and senior British officials on Monday. “But to get to that you could do it sector by sector, and you can do it in a modular fashion. In other words, you can carve out some areas where it might be possible to reach a bilateral agreement very quickly, very straightforwardly.”

Political observers however said it would be very hard to get piecemeal deals through Congress, partly because of the staunch opposition of agricultural and other sectoral lobbies concerned that their interests are being sidelined if they are not included in a broader agreement.

For that reason, trade deals, if they are passed at all, often take years to get through Congress. The existence of a powerful bipartisan Irish American voting bloc, adamant that it will not be complicit in any arrangement that undermines the 1998 Good Friday accord, is almost certain to complicate US-UK trade negotiations still further.

Brendan Boyle, the Democratic co-chairman of the Friends of Ireland caucus, told the Guardian: “The nonsensical utterings of John Bolton should not be taken seriously. He has no role in trade agreements. Zero.”

“I strongly support Speaker Pelosi’s statement today making crystal clear, once again, that protecting the Good Friday agreement is paramount,” Boyle said.

“As the Speaker said: if Brexit undermines the Good Friday accord, there will be no chance of a US-UK trade agreement passing the Congress.”

Forty-five Republican senators have signed a letter indicating willingness to back a trade deal with the UK in case of a no-deal Brexit, but it had no Democratic signatories, fell short of a majority any deal would need, and would not help any agreement overcome more staunch opposition in the Democrat-controlled House.

Fast track authority gives the president the power to negotiate trade deals which Congress cannot amend or filibuster (stall by extended debate). However, the legislature still has the authority to reject such deals outright.