A historic moment when unionist politicians shared a conference room with Sinn Fein for the first time during peace talks in Northern Ireland was recorded in minutes as “something of a damp squib”, newly released Government files have revealed.

The peace talks, involving Northern Ireland political parties and the British and Irish governments – which would eventually lead to the Good Friday Agreement and the establishment of the Stormont powersharing institutions – were taking place in 1997 following Labour’s victory in the UK general election and the calling of a new ceasefire by the IRA.

The Ulster Unionist Party led by David Trimble, then the largest unionist party in Northern Ireland, refused to engage with the talks initially amid disagreement over their structure and concerns over the decommissioning of IRA weapons.

The declassified papers reveal that on September 15 1997, then Northern Ireland secretary Mo Mowlam challenged Sinn Fein’s leadership about an IRA interview in An Phoblact, a republican newspaper, in which the group said it had problems with sections of the Mitchell Principles which committed all parties to exclusively peaceful means, the basis on which the talks had been established.

Minutes of a plenary meeting reveal Ms Mowlam asking “how Sinn Fein could expect people to have confidence in their commitment when it was followed almost immediately by comments by the IRA to the effect that they had problems with a number of the Mitchell Principles”.

Gerry Adams is noted as saying that Sinn Fein and the IRA were not inextricably linked.

The minutes record Sinn Fein negotiator Martin McGuinness telling the plenary meeting that “Sinn Fein could not guarantee that the IRA would support whatever was agreed in the talks”.

A talks summary from two days later reveals that the key event was the “unexpected arrival of the UUP delegation, led by David Trimble”.

The files state that he said he had attended Castle Buildings at Stormont “to confront Sinn Fein rather than negotiate with them”.

By the following week, the minutes reveal that the two parties were in the same room.

They say: “At the afternoon plenary, the Unionists shared the same conference room as Sinn Fein for the first time.

“The purpose of the plenary – the hearing of the UUP’s indictment of Sinn Fein over the Mitchell Principles – was something of a damp squib, particularly as the UUP’s address on their indictment contained no evidence to support their allegations.

“Nevertheless, a historic change had occurred.”

The talks process dragged on into December 1997 without substantive negotiations taking place.

Minutes from December 16 reveal “a long and frustrating day with the parties failing to agree on the key issues for resolution and the format for negotiations following the Christmas adjournment”.

They continue: “The day ended with a meeting of the plenary review.

“There was an atmosphere of despondency and exhaustion around the table as we approached 7pm with the parties facing the reality of their failure to reach agreement.

“A day of little progress. Agreement on key issues before Christmas now appears impossible.”