The SDLP has ruled out a "nakedly sectarian" nationalist electoral pact with Sinn Féin in Northern Ireland during the general election.
The Democratic Unionist Party and the Ulster Unionist Party have agreed to field one unity candidate in each of four general election constituencies in one of the most far-reaching eve-of-poll deals for decades.
East Belfast, North Belfast and the border constituencies of Newry and Armagh and Fermanagh and South Tyrone will be covered.
Party strategists aim to increase the number of unionists at Westminster with the odds of a hung parliament and increased influence for the smaller parties high.
SDLP leader Dr Alasdair McDonnell said: "The SDLP is absolutely opposed to nakedly sectarian electoral pacts."
"We have consistently argued against such pacts and refuse to put party political interests ahead of the need for reconciliation in our divided society."
"I am deeply disappointed that the unionist parties have pressed ahead in this shamefully divisive way."
Sinn Féin had called on nationalists to come together to support progressive politics in response to the unionists' "narrow sectarian and conservative agenda" for the May election.
DUP leader Peter Robinson said: "This is the most comprehensive electoral agreement between our two parties in the last 29 years."
He said he wanted to maximise the pro-Union voice in the House of Commons.
Unionists face a challenge at the polls from Jim Allister's hardline Traditional Unionist Voice, which opposes Sinn Féin's inclusion in power-sharing at Stormont.
Ukip is to run a series of candidates in Northern Ireland which may also erode the mainstream unionist vote.