Peter Robinson at a demonstration at Stormont in the wake of the Anglo-Irish Agreement

In one of a flurry of high-level intergovernmental meetings in 1986, then-British Cabinet secretary Sir Robert Armstrong – the UK’s top-ranking official – told Irish counterparts Mr Robinson was “saying things about independence”.

“We may be tending to treat this as unthinkable and to say ‘they can’t really want it’, but the issue may become more real,” he warned.

Sir Robert was head of the British civil service and chief advisor to prime minister Margaret Thatcher.

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Notes from the meeting at Whitehall, marked “Secret”, were sent back from London to then-taoiseach Garret Fitzgerald in Dublin.

Sir Robert told officials that unionists who feared the recently signed agreement was a precursor to a British withdrawal would prefer an independent Northern Ireland rather than a united Ireland.

“However, they do not appear to have thought out the full consequences of this course – and enthusiasm for it is far less than widespread,” he said.

He told the meeting a lot of unionist thinking “particularly on the part of (Ian) Paisley” was based on the premise that “at some time the British would pull the rug out and that then Northern Ireland would have to go it alone”.

“Paisley wanted to be in a position to blame the British if this happened – and also to be at the top of the heap,” he said, according to Irish official notes of the meeting, just released into Dublin’s National Archives under the 30-year rule.

Concerns of a Rhodesia-style Unilateral Declaration of Independence (UDI) in Belfast were raised repeatedly during several confidential Anglo-Irish meetings that year, the State papers show.

In a meeting with then-secretary of state Tom King at the Irish Embassy in London in February, Mr Fitzgerald said a risk with keeping the Stormont Assembly going at the time was its being used “to declare UDI”.

Mr King, who wanted to keep the forum going “for letting off steam, if nothing else”, agreed that was possible.

“Robinson was certainly thinking in those terms,” he said.

Mr King said there was “real trouble ahead” as Catholics feared a Protestant backlash to the agreement, citing a television programme that showed the UDA and UVF getting organised.

“The British were unhappy about the role of some politicians,” notes of the meeting state.

“Peter Robinson was certainly in touch with the paramilitaries and the British were also extremely dubious about the role of (former UUP deputy leader) Harold McCusker.

“Even Paisley was now looking over his shoulder to the paramilitaries.”

Ken Bloomfield, then head of the Northern Ireland Civil Service, also warned Irish officials in April about an attempted revival of Ulster nationalism.

“Unionists are now beginning to realise that the choice facing them is whether to preserve the Union or preserve their ascendancy,” he said.

“People going for ascendancy may find themselves, logically, on the independence road – however untenable that may be economically and politically.”

Also in April, Mr Fitzgerald met with then-Ulster Unionist Belfast lord mayor John Carson.

One of the mayor’s officials, Alfie Redpath, told the meeting there were close links between some politicians and the paramilitaries, saying the “thuggery is being manipulated with a purpose”.

“Paisley and Robinson seem to be working with a scenario of UDI in mind,” he told the taoiseach.

“They will say, when the violence has reached a certain point, that the only way out is for Ulster to look after itself.

“We could run into a bloodbath.”