Four pro-remain parties in Northern Ireland are to travel to Brussels later this month to meet the EU's chief Brexit negotiator, Michel Barnier.
The leaders of Sinn Féin, the SDLP, Alliance and the Green Party will make the joint trip on 23 September.
They will meet Mr Barnier the following day.
The parties have previously released joint statements expressing concerns about the impact of Brexit on Northern Ireland.
I will travel to Brussels to meet with Michel Barnier alongside SDLP, Alliance Party and the Green Party. Message is clear...The north cannot withstand exclusion from the single market & customs union. We must protect the Good Friday Agreement. The back stop is the bottom line.— Michelle O’Neill (@moneillsf) September 4, 2018
On social media, Sinn Féin's Stormont leader Michelle O'Neill said there was a clear message that "the backstop is the bottom line".
Negotiations between the UK and the EU are ongoing, but the main sticking point remains the Irish border and how it should work after Brexit.
Both sides have agreed they do not want a hard border, but are at odds over how to achieve it.
The backstop is the arrangement which will apply if the Irish border cannot be kept as frictionless as it is now in the context of a wider deal.
The EU has proposed a backstop that would mean NI staying in the EU customs union, large parts of the single market and the EU VAT system.
The UK government has rejected it as a threat to the integrity of the UK, and has suggested a backstop that would see the UK as a whole remaining aligned with the EU customs union for a limited time after 2020.
The SDLP leader Colum Eastwood said the four parties would use the trip to make the case that it is "absolutely essential" that Northern Ireland is protected from a hard Brexit.
"The only way to ensure that our way of life is untouched is to remain within the single market and customs union," he said.
"We do not need a new border in Ireland and we do not need a new economic border in the Irish Sea."