LONDON — Boris Johnson heads to Brussels on Wednesday to meet European Commission President Ursula von der Leyen as both sides make a last-minute push to secure a post-Brexit deal, barely three weeks before huge changes beckon at the end of the year.

Failure to reach an agreement would compound the economic rupture, causing further upheaval for businesses on either side of the English Channel. Johnson’s office confirmed the two leaders would hold a dinner meeting ”to continue discussions on the future relationship between the UK and the EU".

The British prime minister said he hoped that "sweet reason" would prevail. "The situation at the moment is very tricky," he added, though "hope springs eternal".

Von der Leyen said simply in a tweet that she looked forward to welcoming Boris Johnson and continuing "our discussion on the Partnership Agreement".

Following their phone call on Monday, the two leaders said that “significant differences” remained on three key issues — fishing rights, fair-competition rules and the governance of future disputes.

It is a defining moment for Johnson's premiership, whose Brexit cheerleading played a huge part in the vote to leave the EU and then carried him on to Downing Street.

The British government and its negotiators have been renewing appeals for the EU to respect the UK's "sovereignty". The test will be the extent to which Johnson is prepared to compromise to retain access to European markets.

Von der Leyen, meanwhile, will be keen to defend the EU's own "red lines", safeguarding the bloc's single market and protecting it from potentially unfair competition from the UK in the future.

Several EU countries, notably France, have been putting pressure on the Commission not to cede too much ground.

German European Affairs Minister Michael Roth, whose country currently holds the UK's rotating presidency, said, “we are really in a very difficult situation".

Major changes will kick in from the new year with or without a deal, when the UK leaves the EU's Single Market and Customs Union.

Failure to reach a deal on the future relationship would mean tariffs and other costly barriers to trade from January, and leave arrangements in many other areas in limbo, plunging EU-UK relations to a new low.

Tuesday brought some easing of tensions with the news that the EU and UK had reached agreement in principle on implementing new arrangements for Northern Ireland from 2021.

These were set out in the divorce deal that took the UK out of the EU last January, and since then a UK-EU Joint Committee has been working on its implementation. More details are due to be given on Wednesday.

As part of the latest agreement, announced on by its co-chairs, senior UK minister Michael Gove and European Commission Vice-President Maroš Šefčovič, the British government says it will withdraw contentious clauses from Brexit legislation that have caused a furor with the EU.

Measures in the UK Internal Market Bill concerning arrangements for Northern Ireland would have overridden parts of the binding EU divorce deal, breaching international law.

The United Kingdom left the European Union last January but has continued to follow most EU rules during the transition period, which expires on Dec. 31. It will then leave the EU's trading structures and be considered a "third country". — Euronews