Media playback is unsupported on your device Media caption The DUP leader said the decision was about having the "maximum voice for Northern Ireland", as Gareth Gordon reports

Northern Ireland's two biggest unionist parties have agreed a general election pact in four constituencies, including the UK's most tightly contested seat.

The deal between the Democratic Unionist Party and Ulster Unionist Party means there will be a single candidate from the two parties.

The DUP will step aside in Fermanagh and South Tyrone and Newry and Armagh.

The UUP will step aside in East Belfast and North Belfast. Sinn Féin has now called on the SDLP to form a pact.

However, the SDLP told BBC NI's Good Morning Ulster that it was against the idea, calling the unionist move "a sectarian carve-up".

The unionist pact includes the constituency of Fermanagh and South Tyrone, which Sinn Féin held in 2010 by a margin of just four votes.

Despite six months of negotiations, the unionist parties were not able to agree a single unionist candidate for South Belfast.

However, DUP leader Peter Robinson described the unionist pact as the "most comprehensive electoral agreement between our two parties in the last 29 years".

He said it would help to "reduce the number of non-unionists returned" to Westminster.

UUP leader Mike Nesbitt said their aim was to "get the pro-union vote out" in May's general election.

Two of the four constituencies are currently held by Sinn Féin, whose MPs do not take their seats at Westminster under a long-standing policy of abstentionism.

Image copyright Pacemaker Image caption Sinn Féin MP Michelle Gildernew held on to her Fermanagh and South Tyrone seat in 2010 by a margin of four votes

This time, the DUP has agreed to step aside in Fermanagh and South Tyrone, where the party will encourage its supporters to vote for former UUP leader Tom Elliott.

Similarly the DUP will not field a candidate in Newry and Armagh, where the UUP's only Stormont minister, Danny Kennedy, is standing for election.

In turn, the UUP will support former DUP Mayor of Belfast Gavin Robinson in his bid to win back the East Belfast seat from Alliance - Naomi Long unseated the DUP leader at the last general election.

The UUP has also agreed not to stand in North Belfast, where the DUP deputy leader Nigel Dodds faces a challenge for his seat from Sinn Féin's Gerry Kelly.

The DUP leader said grassroots unionists wanted to see his party working with the UUP to "maximise the unionist vote".

"I commend the constituency associations from both parties who have considered the bigger picture and accepted they should stand aside in the greater interests of unionism," Mr Robinson said.

"With a predicted hung parliament, I am calling on all unionists to unite behind these agreed candidates and maximise the pro-union voice in the House of Commons."

The UUP leader said he acknowledged "there will be those who feel disappointed that no understanding was possible in South Belfast".

"The key now is to get the pro-union vote out on 7 May, not just in South Belfast but across Northern Ireland," Mr Nesbitt said.

The DUP is currently the fourth largest party at Westminster, while the UUP has no MPs.

East Belfast Alliance MP Naomi Long said: "The DUP clearly now know that Gavin Robinson cannot win against Alliance in a fair fight, so they have resorted to this anti-democratic move in a desperate attempt to strengthen their position.

"Many UUP voters were already disillusioned by the party's involvement in whipping up sectarian tensions around flags and parades and this decision will only serve to further alienate them.

"This decision has sounded the death knell for the UUP in East Belfast, who have abandoned their voters without gaining any truly winnable seats in return."