The ratification of the UK-EU trade deal was under threat on Thursday after key MEPs said they would reject the pact if Britain pushed through with a plan to delay checks on food going to Northern Ireland.

German MEP Bernd Lange, the head of the European Parliament's trade committee, tweeted that MEPs would vote down the accord if the Brexit divorce deal was broken.

"Still valid," Bernd wrote, pointing to a previous statement that said any violation, or threat to violate, the divorce terms would mean a rejection of the trade bill. 

The warning came a day after Britain unilaterally announced that it will delay until October a grace period under which food and farm products can ship from mainland Britain to Northern Ireland unchecked.

Britain argued that the six-month delay is to provide more time for businesses such as supermarkets and implement the deal's requirements. 

But the delay effectively changes the terms of the January 2020 divorce that found ways to avoid putting a land border between EU-member Ireland and Northern Ireland, which is part of the UK.

To solve the problem, the divorce agreement keeps the territory of Northern Ireland in the EU's customs area, essentially creating a customs border, with checks on food, down the Irish Sea.

In a tweet, Christophe Hansen, the top trade MEP from the right-of-centre EPP party, said the UK was "up to his old games" and shouldn't forget that the deal was still hanging in the balance.

Hansen's comment was specifically addressed to Britain's Brexit minister David Frost, who was in charge of the trade talks that dragged on for nine months until an eleventh-hour deal was reached in late December.

Frost "should be mindful of the fact that European Parliament has not ratified the deal yet and that the full implementation of the withdrawal agreement and its protocol is and remains a red line," he added.

European Parliament is expected to vote on the deal in April.

Hansen said that members of the European parliament handling the UK deal would meet on Thursday with EU Vice-President Maros Sefcovic to discuss the next steps.

Sefcovic co-chairs with Frost a joint EU-UK committee meant to work through issues related to the Brexit treaty. 

The former Slovak MEP on Wednesday said the EU would respond with "legal means" to Britain's announced move, which he said undermined the pact meant to protect peace on the island of Ireland.