EU chief Ursula von der Leyen congratulated Liz Truss on being named Britain's next prime minister Monday, but said London and Brussels must work "in full respect of our agreements".
Truss, the outgoing British foreign minister, has warned that she will push ahead to pass proposed UK legislation to tear up part of Britain's Brexit deal with the European Union.
"The EU and the UK are partners. We face many challenges together, from climate change to Russia's invasion of Ukraine," von der Leyen said, in a congratulatory tweet.
"I look forward to a constructive relationship, in full respect of our agreements," she added, in what will be seen as a reference to the Northern Ireland Protocol.
This clause, part of the agreement under which Britain left the European Union, retains the British province of Northern Ireland inside the EU customs zone and single market.
This was intended to prevent the return of a trade border between the north and Ireland, which remains an EU member, and was signed and approved by Truss's outgoing predecessor Boris Johnson.
But some in Northern Ireland argue that the protocol weakens their link to the UK, and Truss supports legislation progressing through the London parliament to unilaterally dump it.
Brussels has already begun legal action attacking Britain for failure to respect parts of the withdrawal deal, and warned that ditching the protocol could torpedo the whole agreement.
Undaunted by talk of a trade war with the EU, Truss said in mid-August she was "absolutely determined" to ram the bill through in the weeks ahead. She is due to take office on Tuesday.
