His national security advisor Jake Sullivan warned that Mr Biden harbours "very deep" concerns on the issue provoked by Brexit.
Mr Sullivan said the president believes the post-Brexit Northern Ireland Protocol is "critical" to ensuring that the Good Friday Agreement is protected, as Britain and the EU try to resolve the issue of checks in the Irish Sea.
"But whatever way they find to proceed must, at its core, fundamentally protect the gains of the Good Friday Agreement and not imperil that," he added.
"And that is the message that President Biden will send when he is in Cornwall."
Mr Sullivan said the US had been "crystal clear" on the need for the Good Friday Agreement.
He told reporters: "President Biden has been crystal clear about his rock-solid belief in the Good Friday Agreement as the foundation for peaceful coexistence in Northern Ireland.
"That agreement must be protected and any steps that imperil or undermine it will not be welcomed by the United States."
After being asked if Mr Johnson was "taking steps to imperil" the agreement, he added: "I'm not going to characterise that at this point. I'm only going to say that President Biden is going to make statements in principle on this front.
"He's not issuing threats or ultimatums, he's going to simply convey his deep-seated belief that we need to stand behind and protect this protocol."
But not all Tories are happy with the President's intervention. One told Playbook: "He’s so senile that he probably won’t remember what we tell him anyway. Unless an aide is listening I’m not sure he’s going to remember for very long.”
