A “backstop” option keeping the UK aligned with the EU’s Customs Union after Brexit represents the bottom line to safeguard stability, pro-Remain parties in Northern Ireland have said in a rare joint statement.

The proposal was drawn up to avoid a hard Irish border if the EU and UK cannot agree a deal before next year’s withdrawal.

The backstop would mean the UK matching EU customs duties to avoid checks on goods passing between Northern Ireland and the Republic.

A joint statement by Sinn Fein and several smaller parties opposed to Brexit said: “The political parties in the Northern Ireland Assembly wish to reaffirm our common position that we cannot withstand exclusion from the single market or customs union.

“That the protection of the Good Friday Agreement in all of its parts, including North/South and East/West co-operation is critical to maintaining relationships within, and between these islands.

“That the backstop agreed by both the British Government and the EU27 is the bottom line in order to safeguard our political and economic stability now and for the future.”

The main unionist parties, including the DUP whose 10 pro-Brexit MPs are propping up Theresa May’s Government, did not sign the declaration.

Mrs May’s favoured option is a customs partnership whereby the UK would collect tariffs set by the EU customs union on goods coming into the UK.

She faces serious opposition from Brexiteer members of her own Government who back a model relying on technology and advance customs checks to minimise impediments at the frontier.

The EU has expressed doubts about both options.

The Northern Ireland political parties’ statement added: “Sinn Fein, Green Party (cross-community), Alliance (cross-community) and the Social Democratic and Labour Party all share the common position that we should stay within both the single market and customs union and that there should be no hard border on the island of Ireland or between the two islands.

“This is critical to protecting investment, jobs, trade and the hard-won peace.”