Margaret Ritchie said the deal was not credible
The SDLP has rejected a Sinn Fein approach to form an electoral pact in Fermanagh/South Tyrone and South Belfast.
Sinn Fein president Gerry Adams wrote to Margaret Ritchie suggesting they discuss cooperating in specific constituencies in the general election.
The SDLP leader rejected it, saying her the party is against joining forces.
Mr Adams said his party made the proposal after speaking to voters in the constituencies.
"The reason why I put this proposition to the SDLP leader - or at least intended to if she had agreed to meet with me - is because that is what a section of the electorate have been telling us on the doorsteps," he said.
"That's what we've been getting on the doorsteps 'why don't you guys get your act in order and try and ensure the Orange Order doesn't end up choosing who's going to be the MP'."
Ms Ritchie wrote to Mr Adams saying it was not credible for her party to cooperate with such an approach.
In the letter, the SDLP leader said: "I was disappointed to receive such an approach given the fact that both of our parties have roundly condemned as sectarian/tribal, the recent unionist electoral pact in this election.
Ms Ritchie said that co-operation "between our parties or an electoral pact - whatever description you choose - would put my party and yours in precisely the same place as the DUP and UU/Tories."
The development comes after the DUP and the Ulster Unionists agreed to back an independent candidate in Fermanagh and South Tyrone.
On Friday, it was announced that a former chief executive of Fermanagh District Council, Rodney Connor, will stand as an independent.
The DUP 's Jimmy Spratt said the Sinn Fein move showed it feared unionist unity.
"The panic measure undertaken by Gerry Adams shows that the prospect of unionists coming together and putting division in the past fills the enemies of the union with dread and fear," he said.
Anna Lo of the Alliance Party accused Sinn Fein of wanting a "tribal headcount".
"I am deeply annoyed that further attempts have been made to carve up Northern Ireland and, like unionist pact moves, Alliance condemns them," she said.
In the general election in 2005, the combined vote for Sinn Fein and the SDLP was almost 26,000 while the vote for the DUP and the Ulster Unionists was almost 23,000 combined.
Also standing in the constituency, so far, are Sinn Fein's Michelle Gildernew, Fearghal McKinney of the SDLP and the Alliance Party's Vasundhara Kamble.