By Luke McGee, CNN

(CNN) -- Rep. Nancy Pelosi, Speaker of the United States House of Representatives, has warned that Britain will be unable to secure a trade deal with the US if it does anything to undermine the treaty that brought peace to Northern Ireland after decades of violence.

Pelosi's comments came after the UK said it would legislate to override parts of the divorce deal with the European Union in the event that a trade agreement isn't reached. The UK government claims that its Internal Market Bill is designed to ensure that trade between the four nations of the United Kingdom would remain unfettered in the event of a no-deal Brexit.

The UK concedes that it would breach the EU withdrawal agreement; a British Cabinet minister said this week that the legislation would "break international law in a very specific and limited way."

That did not go down well among top Democrats in the US, who fear it could undermine the 1998 Good Friday agreement, which brought peace to Northern Ireland after decades of sectarian conflict.

"If the UK violates that international treaty and Brexit undermines the Good Friday accord, there will be absolutely no chance of a US-UK trade agreement passing the Congress," Pelosi said in a statement on Wednesday.

UK government ministers insist that the legislation would protect, not undermine, the Good Friday accord. The EU vehemently disagreed on Thursday. "The EU does not accept the argument that the aim of the draft Bill is to protect the Good Friday (Belfast) Agreement. In fact, it is of the view that it does the opposite," the European Commission said in a statement. It gave the UK until the end of the month to withdraw the disputed parts of the bill.

The legislation, if voted into law by Parliament, would effectively overwrite elements of the Brexit deal that UK Prime Minister Boris Johnson signed with London last year. Specifically, it would undermine a part of the deal known as the Northern Ireland protocol, which exists to eliminate the need for a hard border between Northern Ireland and the Republic of Ireland, in accordance with the 1998 Good Friday Agreement.

The agreement is of particular interest to US Democrats because of former President Bill Clinton's role in bringing the various sides of the divide in Northern Ireland together.

Pelosi's statement will come as a major blow to the UK, as several prominent Brexiteers have claimed that the ability to sign international trade deals will be the most obvious upshot of leaving the European Union. As a member state and part of the EU's single market and customs union, the UK could not negotiate its own trade deals and instead was represented at the World Trade Organization by a delegate from the EU.

A trade deal with the US has been repeatedly described as the most important of these, given the size of the US economy, the historic relationship between Britain and the US and the fact that the US is the UK's largest single trading partner, despite the two having no formal trading agreement.