Several members of the US Congress have sent a letter to the British Prime Minister Theresa May warning about the potential consequences of the return of a hard border on the island of Ireland.

The letter was signed by 22 US representatives from both sides of the political divide, including Democrats Richard Neal, Joseph Kennedy and Brendan Boyle, as well as Republican Peter King.

It warns Mrs May that if there is a return to a hard border, "additional initiatives, such as a free trade agreement, may be delayed indefinitely."

The members of Congress say they hope Theresa May's government will do everything in its power to avoid the imposition of a hard border.

The letter states that a continuation of the peaceful border "is critical for the future of all the people of both jurisdictions."

Last month, members of the US Congress warned that any return to a hard border on the island of Ireland could threaten a post-Brexit trade deal between the UK and the US.

Democratic Congressman Richard Neal is the Chairman of the House Ways and Means Committee which will oversee any future trade deal between the UK and the US.

He has expressed his concerns about a hard border to the US Trade Representative, the official who advises the US president on trade deals.

Congressman Neal said that while the US would find it desirable to have a bilateral trade agreement with the United Kingdom, he will continue to raise concerns over any return to a hard border.

In January, a resolution was introduced in the US House of Representatives by Congressman Brendan Boyle opposing the return of a hard border on the island of Ireland.

Brexit is likely to dominate the agenda when the Taoiseach Leo Varadkar visits Washington this week for meetings with US President Donald Trump and senior members of Congress.