Sinn Fein has damaged the Good Friday Agreement and played into the hands of the DUP, the SDLP leader has said.
SDLP leader Mark Durkan opposes sanctions against Sinn Fein.
Mark Durkan launched a strong attack on Sinn Fein at his party's annual conference, accusing them of "hiding behind their mandate".
"The truth is so much of what they say and so many of their actions are beneath contempt," he said.
He earlier revealed that Tony Blair had tried to persuade the SDLP to enter a voluntary coalition without Sinn Fein.
The commission which monitors paramilitary activity said on Thursday that senior Sinn Fein members were involved in sanctioning robberies including the £26.5m bank raid in December.
Unionists urged the government to move ahead without Sinn Fein, following the Independent Monitoring Commission report on the Northern Bank raid.
However, the idea of a coalition without republicans, favoured by both unionists and the Alliance Party, has always been stymied by SDLP opposition.
But Mr Durkan stressed his opposition to sanctions against Sinn Fein.
Mr Durkan told BBC Radio Ulster's Inside Politics programme on Saturday, the prime minister tried to persuade him to enter a voluntary coalition in a meeting in Downing Street in January.
"He pressed and pushed us very strongly in the direction of voluntary coalition, exclusion, call it what you will," said Mr Durkan.
"And he was quite prepared to accept those terms as being interchangeable as well."
A Downing Street spokesman said the government had to explore all options, but that "does not mean it has decided on one particular option".
Sinn Fein's Martin McGuinness said Mr Durkan's comments were designed to make his party "relevant" ahead of forthcoming Westminster and local government elections.
More than £26m was stolen from the Northern Bank
"It has always been clear that it was the hope of the British government that the Good Friday Agreement would see the emergence of the Ulster Unionist Party and the SDLP as the dominant parties in any institution of arrangement," he said.
Later, Mr Durkan told his party conference that Sinn Fein believed the "Provisional movement was above the law".
He said the Provisional movement had "let down everybody who made leaps of faith in this process".
"So don't anyone think that the answer is now to ask us to make leaps of fiction," he told delegates.
"When their double speak runs out and their lies aren't just believed, what do they seek cover in? Their mandate.
"But no nationalist voted for bank robberies. No nationalist voted for abductions or families being threatened with death."
Former party leader John Hume told the conference the SDLP would hold onto his Westminster seat at the next general election.