"He has to see the British head of state", adding that other details of the trip had not been finalised.
"But I don't think it would be acceptable for the US President to visit at the current time".
But the PM stressed that the UK-US special relationship meant that she could fully air disagreements, which Britain had with American policy, with the President.
But while Mr Trump may be looking forward to meeting the Queen, his visit to the United Kingdom from July 13 is set to be less than cordial, with protests against the President's domestic and global policies already planned.
"Meeting Her Majesty is the most important thing, because she is head of state".
Trump on Wednesday signed an executive order seeking to replace the family separation policy with one that would detain parents and children together. "When we disagree with the United States", the prime minister said, "we tell them so".
It is thought Mr Trump will meet the Queen in the security of Windsor Castle, which will be closed to the public on the day.
Ms May described images of children in cages as "deeply disturbing".
Saudi Arabia: Deal can be done this week to boost oil supply
Chinese buyers are already starting to scale back orders, with a drop in supplies expected from September. West Texas Intermediate (WTI) crude futures CLc1 were at $66.49 a barrel, up 95 cents, or 1.5 percent.
Buckingham Palace declined to comment.
While plans are still being finalized, Trump's July visit falls on Friday the 13th - a date considered unlucky by some.
"And it is right that we are able to sit down and discuss those with the president".
Sam Gyimah, universities minister, took to Twitter to express his dismay with the policy.
The Mayor of London said: "It's right and proper for us to be active citizens and to march and to lobby and to protest".
"This is the man that rips young children from parents and locks them in cages!" tweeted one user, referring to a practice of separating families of illegal immigrants arriving at the U.S. -Mexico border.
The commander-in-chief has previously been promised a full state visit when Theresa May visited Washington a year ago, but officials have previously confirmed Mr Trump would not be taking up the offer this time around.
Footage and photographs released by the department depict people, including children, housed in large metal chain-link cages within the administration's makeshift shelters.