The European Union has moved a step closer to launching legal action against the United Kingdom government over its decision last week to extend grace periods for businesses in Northern Ireland to adjust to new Brexit trading arrangements.

The UK's unilateral decision to move the end of the period from the end of this month to the fall angered the EU, coming as it has done so early in the building of the new post-Brexit relationship.

It has cast a shadow over wider discussions between the two sides over the status of Northern Ireland, which is now separated from the British mainland, in commercial terms, by an invisible border in the Irish Sea, and caused Ireland's foreign minister Simon Coveney to say the EU was now dealing "with a partner it simply can't trust", while Ireland's former prime minister Leo Varadkar said it was "not the way a respectable, honorable country should behave".

It is not clear how much notice the UK's senior Brexit official, David Frost, gave the EU about the unilateral declaration, which was followed up by him publishing a newspaper opinion piece telling the EU to "stop sulking" over Brexit, which has not gone down well across Europe.

The Guardian newspaper quoted a UK government official as saying the EU had been informed at an "officials level", adding "these measures are lawful and consistent with a progressive and good-faith implementation of (Brexit agreement sections relating to Northern Ireland)."

At a meeting of European leaders on Tuesday, European Commission Vice-President Maros Sefcovic explained the options available, and Irish state broadcaster RTE says a legal letter of formal notice could well be issued to the UK in the coming days.

The Irish Times reports that the weight of the United States government may also be brought into play by the EU.

Sefcovic and Coveney are in Washington this week and will speak to the influential Congressional Friends of Ireland caucus about what is happening.

During the time of former US President Donald Trump, before the Brexit Agreement was secured ahead of Christmas, much of the case in favor of Brexit was pinned on the hope of a free trade deal between Britain and the US, something that did not happen on Trump's watch.

His successor in the White House, Joe Biden, is widely perceived as being far more friendly toward Europe, and has already spoken to Commission President Ursula Von Der Leyen about working together on a combined strategy to fight the novel coronavirus pandemic.

Even before the election took place, Biden said any possible trade deal between the US and the UK was dependent upon the 1998 Good Friday Agreement, which ended the decades of violence known as the Troubles, being upheld, but critics have accused Britain's current tactics over Northern Ireland of putting it in jeopardy.