Sinn Fein has announced it will not stand in three constituencies in Northern Ireland to help other pro-Remain candidates defeat DUP Brexiteers.

The party is standing aside in south and east Belfast and North Down - a move that puts the republicans in the highly unusual position of explicitly urging supporters to vote for a unionist, in the form of incumbent North Down MP Lady Sylvia Hermon.

Announcing the move in Belfast, party president Mary Lou McDonald also urged voters to back the SDLP's Claire Hanna in South Belfast and Alliance Party leader Naomi Long in East Belfast.

Sinn Fein will not contest East Belfast, South Belfast & North Down in #GE2019



I appeal to the electorate to vote progressive, pro-remain candidates that have a positive vision for our future.



Make your voice heard, and make it count. pic.twitter.com/HiC7JKhDU8 — Michelle O’Neill (@moneillsf) November 4, 2019

The move came hours after the SDLP announced it would stand aside in three seats as well, again to facilitate Remainers in their efforts to beat DUP candidates.

The nationalist party will not put forward candidates in North and East Belfast and North Down.

The move in North Belfast will give Sinn Fein candidate John Finucane a better chance of unseating DUP deputy leader and long-standing MP Nigel Dodds, who has a majority of just over 2,000.

Outlining Sinn Fein's decision on Monday afternoon, Ms McDonald said: "In many ways this is a once-in-a-generation election, the stakes are very high in this election.

"People have a fundamental choice to make - to vote for a positive, inclusive future or to turn their backs on that and to back candidates who have been the architects of Brexit and who have acted very, very deliberately against the democratic wishes of people here in the north and more fundamentally against the economic and social interests of citizens who live here."

She added: "The reality is we are asking people to come out and vote for those pro-Remain candidates. We believe that is the right and progressive thing to do."

Ms McDonald said it sat comfortably with her to urge Sinn Fein supporters to vote for a unionist.

"It sits very comfortably with me to ask and invite voters to thoughtfully do the right thing," she said.

"And in this case it means defying hard Brexiteers, the likes of Nigel Dodds, the likes of the DUP candidates who have very, very recklessly acted against the interests of everybody.

"Whether you call yourself a unionist or a nationalist, a republican or a loyalist, we actually have many, many interests in common."

The SDLP decision not to field a candidate in North Belfast for the first time in the party's history has prompted claims of hypocrisy from political rivals, given the SDLP's vocal criticism of Sinn Fein's Westminster abstentionist policy in other battleground constituencies, such as Foyle.

Former SDLP councillor Mairia Cahill resigned from the party in protest at the move, accusing the party of facilitating a "sectarian headcount".

The SDLP said it had taken an "extraordinary decision" for an "extraordinary election".

"We have taken a unilateral decision not to stand candidates in North Down, North Belfast and East Belfast," said the party.

"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 Ulster Unionist Party has already stepped aside in North Belfast to give Mr Dodds a better chance of winning.

The UUP decision also proved controversial, as it came only a week after incoming leader Steve Aiken pledged to run in all 18 constituencies in Northern Ireland. He has been accused of a major U-turn before he has even been officially elected as leader.

Police are investigating alleged paramilitary threats made to the party in the wake of Mr Aiken's initial pledge.

The absence of SDLP and Sinn Fein candidates in East Belfast is designed to give Ms Long a better chance of defeating incumbent DUP MP Gavin Robinson, while in North Down the parties hope independent unionist and pro-Remain MP Lady Sylvia can see off a DUP challenge.

Sinn Fein's standing aside in South Belfast will strengthen SDLP candidate Ms Hanna's chance of beating sitting DUP MP Emma Little-Pengelly.

At the weekend the DUP announced it would continue its recent policy of not contesting Fermanagh and South Tyrone - a move that will assist an Ulster Unionist candidate in efforts to unseat Sinn Fein MP Michelle Gildernew.

The cross-community Alliance Party has made clear it has no role in the electoral manoeuvres and will not engage in any pacts.