May first told The Sun that the "relationship with America is our deepest and most important defense and security relationship."
Trump has drawn harsh criticism for taking a dig at Khan in the aftermath of Saturday’s attack, and May's remarks add strength to the argument that he crossed a line with his apparent critique of the mayor Sunday.
"At least 7 dead and 48 wounded in terror attack and Mayor of London says there is "no reason to be alarmed!" Trump tweeted then.
But the mayor's remarks about being alarmed, which focused not on the subject of terror but on an increased police presence in the wake of the attack, were taken out of context.
"Londoners will see an increased police presence today and over the course of the next few days. No reason to be alarmed," Khan said while addressing residents.
Not mentioned by Trump was that Khan also warned that the threat level in London remains severe.
“Severe means an attack across the country is still highly likely, and so we have all got to be vigilant,” Khan said.
A spokesperson for Khan on Sunday called Trump's remark "ill-informed" and said it took the mayor's words out of context.
The White House has downplayed the clash of words between Trump and Khan.
“You should be alarmed,” Eric Trump told ABC News, referring to terror attacks like the one that took place in London over the weekend. “Because this has become the new norm. And it's not right. And we, as a society, especially as Americans, better do something about it.”