Letter signed by 52 of the Northern Ireland Assembly's 90 members says it "flies in the face" of the views of most people in the province.
The prime minister has also defended a new law which would allow the UK to unilaterally rip up the Northern Ireland Protocol.
Assembly unable to function amid fall-out from post-Brexit trading arrangement.
Sinn Fein is now the largest party in Northern Ireland for the first time in Stormont's history.
Sinn Fein's position as the largest party means a poll on the reunification of Ireland is now far more likely.
What you need to know as polls suggest Sinn Fein is close to a milestone in its quest for a united Ireland.
Lord Hayward has predicted a bad night for the unionist parties on May 5.
A bus was hijacked and set on fire and a press photographer was assaulted in the latest disorder.
Everything you need to know as Sinn Fein's surge smashed the country's traditional two party politics.
Left-wing nationalist party appears to be out performing an exit poll that predicted a three-way dead heat.