The Northern Secretary, Mr Mandelson, has said it would be a mistake to read too much into the result of the South Antrim by-election.
He said the result indicated "some unease within the unionist community" but the outcome was primarily due to the choice of candidate fielded by the Ulster Unionist Party.
"If you infer anything from this result, it is that faced with the choice between two anti-agreement unionists, many unionists chose to stay at home and not vote at all," he said.
Mr Mandelson expressed his support for Mr Trimble but said the result would not drive him to alter British policy. "I think he is an excellent First Minister for Northern Ireland, but the overriding and governing imperative for me and my colleagues is to make sure that an agreement that has been made sticks, that it is implemented and it is implemented in a balanced and fair and reasonable way."
Mr Mandelson said he did not believe the election demonstrated a change in unionist opinion.
"What we've seen in this result, set against the result of the Assembly elections in 1998, is that an evenly balanced unionist vote, pro- and anti-agreement is remaining broadly the same."
The Deputy First Minister, Mr Mallon, said the vote was a setback that must be overcome. "We all must try and ensure that whatever knocks we take - and this is a knock - that we keep going and keep working towards the goal of providing the best possible administration for all of the people of the North of Ireland."
Mr David Ford MLA, the Alliance party's candidate for the seat, said that had the UUP "run a pro-agreement candidate, they would have won the seat". He said the lesson for the Ulster Unionists was that they "actually have to start standing up for what they believe in". Were they to do so, "William McCrea's victory may prove to be short term."
The Sinn Fein leader said: "The majority of unionists who supported the Good Friday agreement were given no choice, and were, in fact, demotivated by the confusing signals being sent by the UUP."
Mr Adams insisted the result was no excuse to water down nationalist goals. "I want to make it abundantly clear that that is not acceptable. Nationalist rights and entitlements cannot be filtered through a unionist prism," he said.