The UK and the European Union appear to be on the brink of completing a new Brexit deal for Northern Ireland.
A No 10 source describes the negotiations as "positive".
King Charles had also been due to meet the president of the European Commission in the UK on Saturday, the BBC understands.
Multiple sources say the visit has been cancelled due to operational reasons unconnected to the political talks.
The planned meeting between the King and Ursula von der Leyen, originally reported by Sky News, was not part of the negotiations between the UK and the EU.
But the fact a meeting was planned is significant as it appears to indicate a Brexit deal was about to be done - and publicly presented - while Ms von der Leyen was in the UK.
It is not known when the European Commission president will now come to the UK. luca168