London prepares to pump up the volume in Brexit negotiations on Northern Ireland

Irish officials say they are aware that Mr Frost will tomorrow unveil a Brexit update referred to as a “Command Paper”, and this will span a range of core issues. These include animal, plant and food checks, customs and VAT arrangements, treatment of manufactured goods, Northern Ireland trade with Britain, state aid rules and an expected criticism of the EU stance.

Reuters cited sources in Brussels and London as saying the UK Brexit Minister is expected to threaten to invoke Article 16 of the Northern Ireland Protocol which allows either party to take unilateral measures if applying the protocol "leads to serious economic, societal or environmental difficulties that are liable to persist".

The threat would increase tensions between the sides as efforts are being made to use extended grace periods due to end in October to find more lasting compromises. Attitudes are hardening in Brussels and key EU capitals with a view that London did this deal late last year, and it now has international treaty status which must be honoured.

The Democratic Unionist Party leader, Jeffrey Donaldson, said yesterday it was time EU leaders admitted the protocol “has failed” and is creating “very substantial trade problems”. He was speaking after a virtual meeting with EU Commission vice-president Maros Sefcovic, and he told the BBC “both the UK government and the EU must now renegotiate”.

Foreign Affairs Minister Simon Coveney also yesterday held talks with Mr Sefcovic, who heads the EU Brexit delegation. His spokesman said he would see what Mr Frost says later this week before making any comment – but Mr Coveney has consistently said London must honour its obligations and the EU can help with an effective application of the rules.

Both Dublin and Brussels are determined that there can be no reopening of the Northern Protocol or the Brexit deal which was finally done last Christmas Eve and some of which took effect on January 1. There have been a series of disputed extensions to grace periods on detailed rules.

Yesterday Mr Frost said the element of the Brexit deal which relates to trade with Northern Ireland is not sustainable and Britain is keeping all options on the table over how it might act.

"We all know the protocol is not sustainable in the way it's working at the moment," he told a parliamentary committee, saying barriers on goods moving between mainland Britain and Northern Ireland needed to be removed.

"All options are on the table."

Since the UK left the EU on January 1, Prime Minister Boris Johnson unilaterally delayed the implementation of some provisions of the protocol and Mr Frost has said the protocol is unsustainable. Mr Frost is insisting on a bespoke veterinary deal based on equivalence which would remove the need for controls on goods crossing from Britain to Northern Ireland.

Britain is arguing there should be a more flexible approach to agri-food rules to limit the impact on everyday lives, and will spell out clearly what the options and risks are. The 1998 Good Friday Agreement guaranteed an open Irish land border to help safeguard peace, free trade and travel on the island.

But that became a problem after the 2016 Brexit vote. The EU could not close the land border between Northern Ireland and Ireland, but feared it could become a backdoor into the EU's single market.

The result was the protocol, which keeps Northern Ireland in the EU's single market for goods and applies EU customs rules at the North's ports.