British Prime Minister Theresa May and US President Donald Trump wait for a meeting at the Palace Hotel in New York.

The upcoming visit from President Donald Trump to the U.K. could bring fresh instability to Prime Minister Theresa May, at a time when political pressure is building within her own party.

May reached an important agreement Friday with her ministers that paves the way for future negotiations with the EU, with the official Brexit date due in March next year. The so-called Chequers agreement suggested the U.K. should try to keep the closest possible relationship to the one it currently enjoys with the bloc, allowing smooth trade across the English Channel.

But nothing is set in stone yet and the two negotiating teams have a lot of ground to cover if they want to reach a deal before the departure date. Furthermore, Foreign Affairs Minister Boris Johnson and David Davis, the chief Brexit negotiator, resigned in the aftermath of the agreement, raising further doubts about what’s going to happen to the U.K. and its links with the European Union.

Johnson could even meet Trump during his visit to the U.K., despite him no longer being a sitting cabinet member. Trump told reporters Tuesday “Boris Johnson is a friend of mine. He’s been very nice to me, very supportive. I like Boris Johnson, and maybe we will get a chance to talk to him when we get over there."

A public backing of Johnson would no doubt prove challenging for May.

“Trump might put pressure on the British cabinet to reconsider May’s Chequers declaration,” Rem Korteweg, head of the Europe in the World unit at thinktank Clingendael, told CNBC via email Monday.

“This could complicate things for May,” he added.