A move to withdraw support for the Good Friday Agreement by a body that represents loyalist paramilitaries should be "deplored", Lord Trimble has said.
The former UUP leader, one of the architects of the peace deal, criticised the Loyalist Communities Council's (LCC) actions.
The LCC wrote to Prime Minister Boris Johnson and Taoiseach Micheal Martin this week stating that paramilitaries were temporarily withdrawing their backing over the protocol.
The letter was signed by the chairman of the LCC, David Campbell, formerly a senior figure in the UUP who was chief of staff to Lord Trimble when he was First Minister.
On Friday, referring to the letter, Lord Trimble told the Belfast Telegraph: "That is to be deplored. They must realise that by repudiating the Belfast Agreement of 1998, they are also kicking out that which they and their communities depend on."
Ulster Unionist grandee Lord Empey also criticised the LCC's move.
He told this newspaper: "I regret that the support for the agreement has been withdrawn at this stage because I believe that is the key to constitution guarantee and indeed that is why those parties supported it back in 1998, and it was endorsed by a referendum at that stage.
"I understand the frustrations that people feel. We all share them at the moment but I think in a very fast moving situation that we are in, with the implications for the Protocol etc ... it does require a strategic approach from all of unionism, in my opinion."
Earlier, a former No 10 chief of staff warned the protocol undermines the guiding principles of the 1998 accord.
Jonathan Powell, who worked for Tony Blair and was involved in negotiations that forged the agreement, said he understood loyalist anger at the protocol, which has led to an economic border in the Irish Sea.
Mr Powell criticised the Government's move to unilaterally delay the full implementation of the protocol, accusing it of unnecessarily "poking the EU in the eye" when it needed to work with the bloc to find solutions to the trade problems.
He said the Government's actions in extending Protocol grace periods without EU approval were counterproductive at a time when it was important to build relationships, rather than damage them - particularly in regard to Ireland.
On Friday the EU said the UK's move could make it "more tricky" to find permanent solutions to trade disruption.
The European Commission did not rule out ultimately agreeing to the extension of protocol exemptions announced by the UK, but it said those decisions should only be taken on a joint basis.
The Commission also confirmed that preparatory work on its legal action against the UK was continuing.
Commission spokesman Eric Mamer said if the EU was to demonstrate flexibility on protocol issues, the UK had to demonstrate it was prepared to implement commitments it had already agreed to.
In an interview with Friday's Financial Times, European Commission vice president Maros Sefcovic - who is responsible for overseeing the implementation of the agreement - said the European Commission is now working on "infringement proceedings" against the UK.
But First Minister Arlene Foster accused the EU of belligerence.