Sinn Féin will not run candidates in three of Northern Ireland’s 18 constituencies to boost the chances of other anti-Brexit candidates in next month’s election.

The party announced on Monday it would stand aside in South Belfast, East Belfast and North Down to give pro-remain candidates an edge against Democratic Unionist party (DUP) candidates.

“You can call this a pact. You can call it what you wish. The reality is that we’re asking people to come out and to vote for those pro-remain candidates,” said Mary Lou McDonald, the Sinn Féin leader.

The announcement followed a similar promise hours earlier by the moderate nationalist Social Democratic and Labour party (SDLP) to stand aside in North Belfast, East Belfast and North Down.

The pact could help Sinn Féin’s John Finucane take the North Belfast seat held by the DUP deputy leader, Nigel Dodds, help the SDLP’s Claire Hanna topple the DUP’s Emma Little-Pengelly in South Belfast, and help Lady Sylvia Hermon, an independent unionist MP, fend off a DUP challenge in North Down.

McDonald said she had no problem urging Sinn Féin supporters to vote for Westminster’s only pro-remain unionist. “It sits very comfortably with me to ask and invite voters to thoughtfully do the right thing.”

Pro-remain forces needed to unite to block reckless Brexiters, she said. “Whether you call yourself a unionist or a nationalist, a republican or a loyalist, we actually have many, many interests in common.”

The SDLP said standing down in three constituencies was an “extraordinary decision” for an “extraordinary election”. “Our first priority is to elect pro-remain MPs to vote against Brexit and Boris Johnson but removing pro-Brexit MPs in Belfast is also critical.”

The decision by both nationalist parties to stand down in East Belfast will bolster the effort by Naomi Long, the leader of the non-aligned Alliance party, to wrest the seat from the DUP. Few expect Alliance to reciprocate by standing down in South Belfast or North Down.

Commentators said Brexit facilitated the Sinn Féin/SDLP accord but that it was driven by raw electoral calculation. “It may be hung on remain vs leave, but the ‘pacts’ are just old-fashioned orange vs green,” said the commentator Alex Kane in the Belfast Telegraph.

Last week Steve Aiken, the pro-remain incoming leader of the Ulster Unionist party (UUP), briefly upended the green/orange convention by ruling out an electoral pact with the DUP, putting Dodds, an ardent Brexiter but fellow unionist, at risk.

Aiken U-turned after unionists inside and outside the UUP protested and suspected loyalist paramilitaries threatened retaliation against his party. “It is better to elect Nigel Dodds in North Belfast and hold him to account for his promises on the union than facilitate the election of an abstentionist Sinn Féin MP who still cannot condemn IRA violence,” said Aiken.