The diplomatic position was created in 1995 with the appointment of former US senator George Mitchell, who went on to chair the talks that led to the Good Friday Agreement.

But Mr Tillerson, in a letter reportedly sent to US senator Bob Corker on Monday, said the post could now be “retired” because “the 1998 Good Friday Agreement has been implemented with a devolved national assembly in Belfast now in place”.

The letter did not make any reference to the collapse of Northern Ireland’s power-sharing institutions in January 2017.

More than two dozen members of Congress have since written to Mr Tillerson to urge him to reverse his decision.

A letter seen by The Irish Times, signed by at least five Republicans as well as Democrats, notes that “nearly 40 million Americans of Irish descent have begun to speak out on this issue”.

It goes on to state that “the signing of the Good Friday agreement in 1998 represents one of America’s most successful foreign policy accomplishments in recent memory”.

Foreign Affairs Minister Simon Coveney has also said he would like to maintain the envoy to Northern Ireland.

A group of US politicians are planning to Northern Ireland next month to examine the implications of Brexit on Northern Ireland.