David Trimble was said to have been “extraordinarily rude” during a meeting with Tony Blair and Bertie Ahern in June 1999.

The then UK prime minister and the then taoiseach held meetings in Castle Buildings on June 25 1999 with all the Northern Ireland parties, outlining actions to be taken as part of the peace process.

An Irish civil servant’s briefing note of the encounter, released in this year’s State Papers files, outlined a tense exchange where Mr Trimble accused Mr Blair of laying a “crude trap”.

The meeting was held at a time when there was enormous political pressure on Mr Trimble to ensure paramilitary decommissioning after signing his Ulster Unionist Party (UUP) up to the Good Friday Agreement peace deal.

At each of the meetings with the various pro-agreement parties, the two premiers sought support for three principles on decommissioning.

Although there was said to be “no difficulty” with the first two principles, the third – which stated decommissioning was “to be carried out in a manner determined by the Independent Commission on Decommissioning” – posed issues for the UUP.

Concerns were raised about whether “manner” meant the commission could decide the timing of decommissioning, or the way in which paramilitary arsenals were disposed of.

A briefing note compiled by senior civil servant Dermot Gallagher said that Mr Trimble’s arrival at the meeting with a delegation of 10 was “a clear reflection of the lack of trust within the party and of Trimble’s limited room for manoeuvre”.

Those at the meeting included Jeffrey Donaldson, who said “manner” was “vague and imprecise” and clarity on it was needed.

Mr Trimble was noted as arguing that the third principle “related to modalities and methodology and not timing”, while Mr Blair said the party was reading “far too much into the issue”.

He said that they could agree to the principles on the basis of an inclusive executive, and that the word manner “related to methods and modalities”.

While leaving the meeting, Mr Trimble was said to have had “a very difficult discussion” with two British officials in which he disagreed with the preamble to the three principles.

This stated “although there is acknowledged disagreement about their implementation, all parties to the Good Friday Agreement are committed to the following principles”.

Mr Trimble expressed concern that this did not safeguard his concerns about timing; Mr Blair invited Mr Trimble and his delegation to discuss the issue further.

“The tone and approach of Trimble at this session was extraordinarily rude,” Mr Gallagher wrote.

Mr Trimble stated his concern on being bound by the timing of decommissioning set by the commission and accused it of being a “crude trap” that he had “no intention of walking into”.

The note stated that Mr Blair “resented the accusation”, and said that the decommissioning body could not “unilaterally” decide on timing.

The note then stated: “Trimble responded by saying that this was ‘not consistent with what’s there in black and white’, adding that ‘we’ve had crude tricks like this played on us before’.

“When the Prime Minister responded with ‘not by me’, Trimble added ‘not yet’,” to which Mr Blair replied “in exasperation” that he was trying to help.

The meeting concluded with an agreement to change the preamble to state “disagreement about timing and implementation”.

The Progressive Unionist Party (PUP) raised concerns with the second principle that set a deadline of May 2000 for decommissioning, and due to delays in setting up the executive, said a new deadline should be set.

Leader of the Ulster Democratic Party Gary McMichael said it accepted working towards a decommissioning deadline of May 2000, and said that loyalist paramilitaries were “very much watching what the IRA would do”, and as a result they were not able to say that the UDA would decommission.

PUP leader Hugh Smyth said that if the executive was working well once set up, “some slippage might be accepted and the May 2000 deadline extended”.

PUP MLA David Ervine said that the focus should not be on decommissioning, but on ending the war, and said relations would “never be the same” if the Good Friday Agreement went down.

Sinn Fein, the SDLP, Alliance and the Women’s Coalition agreed to the three principles.

The material can be viewed in the National Archives in file 2023/154/4