Ms Truss’s letter to Mr Johnson is understood to have set out two possible options – collapsing ongoing negotiations and triggering Article 16 now, or holding off using the safeguard clause and instead deploying an economic stimulus package to help firms losing out because of the implementation of the protocol.

Talks with Maros Sefcovic, Ms Truss’s EU opposite number, would continue as part of this plan, with Article 16 being held in reserve.

The clause allows either side to take unilateral “safeguard measures” if the protocol causes “serious economic, societal or environmental difficulties”.

The UK insists the legal threshold for triggering the clause was met months ago, and Ms Truss used an article for The Telegraph in January to insist she would use it if it became necessary “to ease acute problems”.

Ms Truss, who took over the role of post-Brexit negotiator from Lord Frost in December, has since expressed private frustration at EU intransigence.

‘Sucked in by Remainer lobby’s propaganda’

But some Brexiteers fear that she does not have the “bandwidth” to be an effective negotiator on the protocol at the same time as working on the response to Russia’s invasion of Ukraine. Last week she was in the US for talks with Antony Blinken, the Secretary of State, and Jake Sullivan, the National Security Advisor.

On Saturday night, a Conservative source claimed: “The Foreign Secretary has been sucked in by the Remainer lobby’s propaganda. We should be using the Article 16 safeguards to defend the integrity of Northern Ireland, unite its communities, restore political confidence and end trade distortions.  

“What we don’t need is to treat Northern Ireland as a limp limb of the Union by giving it patronising state handouts.”

A senior backbencher added that MPs were concerned the Government “don’t seem to have the bandwidth to deal with both the situation in Ukraine and the Northern Ireland protocol and, even if they did, they don’t have the will”.