The SDLP has warned the Northern Secretary that it will not sit on the new policing board if legislation before the House of Commons fails to implement the Patten report.
The North's Deputy First Minister, Mr Seamus Mallon, said: "If he gets it right, we will nominate to the policing board. If he doesn't, we won't."
Mr Mandelson will open the debate on the Police Bill at Westminster this afternoon. The second reading is expected to be passed tonight, and the committee stage is due to be completed by the end of July. The growing tension and rivalry between the two main nationalist parties in the North on the issue was reflected in a Sinn Fein claim that the SDLP had taken a "self-serving" and "opportunistic" approach to the campaign over the legislation. The Sinn Fein vice-president, Mr Pat Doherty, said the SDLP had made a blatant attempt to portray itself as the only party opposing the Bill.
Amid expectations that the Northern Secretary would try to allay nationalist concerns without alienating unionists, the Police Ombudsman for Northern Ireland, Ms Nuala O'Loan, expressed optimism after a meeting with the Northern Ireland Office that her concerns about the Bill would be addressed.
A number of human rights groups, including Amnesty International, said there were serious failings in the proposed legislation.
Amnesty said the Bill undermined many of the good recommendations in the Patten report and weakened a key tenet of the document, namely, its focus on police accountability.
Mr Martin O'Brien of the Belfast-based Committee on the Administration of Justice said it was inconceivable that the British government would want to see a watering-down of the fundamental aspects of the Patten report. "We will all be campaigning to ensure that that does not happen."
The SDLP had two lengthy meetings on the Bill last week with Northern Ireland Office ministers, one lasting five hours with the Security Minister, Mr Adam Ingram. A second discussion with the Northern Secretary and Mr Ingram lasted 3 1/2 hours.
The party expressed its concern about the independence of the policing board, arguing that the board should have the power to inquire into events which took place before its establishment, and that the Code of Ethics should be decided by the policing board, not the Chief Constable.
Asked if he expected the Bill in its final form to be acceptable, Mr Mallon said: "I hope it is. We can do no more on it. Our party has spent 30 years trying to get this right."
Mr Mallon said he had spent most of his adult life campaigning on the issue. He had "not a shred of regret" about his party's early acceptance of the Patten report.
Sinn Fein said the SDLP was not the only organisation in opposition to the Bill. Apart from his own party's efforts, Mr Pat Doherty said, there had been a broad spectrum of opinion lobbying against the legislation, from the Catholic Church to Washington.
"As far as Sinn Fein is concerned Patten doesn't go far enough, but it would represent a beginning. Any dilution of the Patten proposals such as what we see in the Mandelson Policing Bill is clearly unacceptable," Mr Doherty said.
He announced that he was on his way to London with his party colleague Mr Gerry Kelly to campaign for the full implementation of the Patten report "as the minimum required if we are to see a new beginning to policing".