Mr David Trimble said in the House of Commons yesterday that there were a number of issues on which he had disagreed with Mr Peter Mandelson, particularly over policing.
Speaking during Northern Ireland questions, Mr Trimble said: "The proposals - particularly with regard to the change of the name of the RUC and the failure to recognise the service and the sacrifice of the members of the RUC - have caused very deep hurt within the community as a whole.
"Furthermore, the effectiveness of the police force is being undermined by the continuing uncertainty that exists with regard to certain aspects of policing.
"The sooner we get a closure on this issue the better and this requires now for nationalists to themselves take up their responsibilities for society," he said.
Mr Mandelson replied: "The government is conscious, and I have only been too conscious personally, that some of the changes involved are painful for some people in Northern Ireland. But I can say very firmly and very clearly, the sacrifices that have been made by the RUC will never, ever, be forgotten."
He added: "I do echo your remarks. It is incumbent on all sections of society to move forward in support of the police service and to encourage young men and women right across the community in both traditions to become part of this unique opportunity for policing in Northern Ireland."
The Shadow Northern Ireland secretary, Mr Andrew Mackay, told him: "The police reforms are in real danger of being all pain, no gain, particularly if the Irish Government, SDLP and Sinn Fein continue to refuse to recommend to young Catholic men and women that they join the new police force."
Mr Mandelson said the greatest gain was the development of a police service which won the trust and respect of both sides of the community. This would allow the police to tackle crime with the active support of both sides of the community "rather than the hostility or the sullen acquiescence that we have seen in the past".
Mr Mandelson went on to say: "One of the government's and one of my own personal proudest achievements has been the contribution I have made in introducing and in the passage of the Police Act of Northern Ireland that is leading to the creation of the new police service in Northern Ireland."
Mr Mackay said there was a widespread belief that the government was putting political pressure on the armed forces to move towards demilitarisation in Northern Ireland.
The Northern Ireland Security Minister, Mr Adam Ingram, said political pressure was never exerted on the government's security advisers.
Their "first-class advice" was always sought and responded to before any course of action was undertaken.
Conservative MP Mr Eric Forth asked: "For how much longer are the government going to fool themselves, and, worse still, the British people, over decommissioning?"
The Rev Martin Smyth, of the Ulster Unionist Party, said: "Is it possible that we perhaps have been listening to our own spin and the republicans have rumbled it?"
But Mr Ingram said it was more a matter of looking at the "very hard reality" of the situation.
"No one said it was going to be easy, no one said there were not going to be risks attached but all of that is a reality we are dealing with," he said.
Earlier, opening questions on Northern Ireland, Conservative MP Mr Julian Brazier said: "In view of the deadlock in the negotiations and the uncertain, to say the least, security position, would you as your last official action in this chamber do a little more to lift the shadow from this brave, impartial and disciplined service still known as the Royal Ulster Constabulary?"
Mr Mandelson replied: "The government's aim throughout this process is to develop a modern police service in Northern Ireland which is both effective and representative of the community it serves, and commands the widespread support and confidence it needs to do its job effectively. The changes being implemented will, in the government's view, achieve this.
"The government has made clear that it is committed to the radical vision expressed by Patten which in our view will contribute as much, if not more, than anything else to the longterm peace and stability for all the people in Northern Ireland."
Mr Nicholas Palmer, a Labour member of the Northern Ireland Affairs select committee, said the political parties in Northern Ireland had to be urged not to use the Secretary of State's resignation as an excuse for further delay in the peace process.
Another Labour MP, Mr Dale Campbell-Savours, said Mr Mandelson's "regrettable" resignation was a warning to all MPs of what could happen "when the press want a politician's head on a platter".
"Can I express the hope that those same editors who have called for his resignation will now turn their attention to writing articles congratulating him for the work that he did in Northern Ireland in resolving the problems of conflict?" he added.
Mr Ingram said the Irish News, the Belfast Telegraph and the News Letter had taken a "very balanced and constructive" approach to the issue. "I would hope other parts of our media could respond to that approach adopted by the newspapers in Northern Ireland," he concluded.