US President Donald Trump's special envoy to Northern Ireland, Mick Mulvaney, has warned against creating a "hard border by accident" on the island of Ireland.

British Prime Minister Boris Johnson is proposing new legislation that would break the Northern Ireland Protocol of the Brexit divorce treaty that seeks to avoid a physical customs border on the island of Ireland.

"The Trump administration, State Department and the US Congress would all be aligned in the desire to see the Good Friday Agreement preserved to see the lack of a border maintained," Mr Mulvaney said in an interview with the Financial Times.

His remark comes two days after US Democratic presidential candidate Joe Biden warned Britain that it must honour the 1998 agreement as it withdraws from the European Union or there would be no separate US trade deal.

Latest Brexit stories

Meanwhile, the European Commission chief has said she is "convinced" a trade deal remains possible with the UK, but called Mr Johnson's attempt to override the Brexit treaty an "unpleasant surprise".

Ursula von der Leyen, in comments made to reporters yesterday, said Downing Street's controversial UK Internal Market Bill had "distracted very strongly" from the two sides being able to secure fresh trade terms before the looming deadline.

The post-Brexit transition period, during which relations between the EU and the UK have remained static, is due to end after 31 December and leaders on both sides of the Channel have warned that an agreement is needed by October if a deal is to be ratified in time for the start of 2021.

With the cliff-edge only a month away, Mr Johnson has faced criticism domestically and on the world stage for pursuing legislation that would defy the Withdrawal Agreement brokered with the EU last year, breaking international law in the process.

Mr Johnson was forced on Wednesday to agree to table an amendment to the Internal Market Bill, giving MPs a vote before the government can use the powers related to Northern Ireland that would breach the treaty.

But the compromise has not seen Brussels back down, with Eric Mamer, chief spokesman for the European Commission, telling a press briefing that its position had not changed and it still wanted the offending clauses to be withdrawn from the legislation.

Despite the wrangle over the bill, which has been derided by every living former prime minister, scores of senior Tory backbenchers, Mr Biden and Brussels, Ms von der Leyen said she remained sure that consensus on a future partnership with the UK could be reached.

It comes after she used her annual State of the Union address to the European Parliament on Wednesday to warn Mr Johnson the UK could not unilaterally set aside the Withdrawal Agreement.