Trump told Britain's ITV television in a wide-ranging interview that the military had strict rules regulating the use of prescription drugs.

"They take massive amounts of drugs, they have to, and you are not allowed to take drugs, you are in the military, you are not allowed to take any drugs," Trump said.

"And they have to, after the operation they have to, they have no choice."

The ban went into force in April following a protracted legal battle that went all the way to the US Supreme Court.

The Pentagon says the restrictions are not a blanket ban. But they do bar an overwhelming majority of people who identify as transgender from enlisting.

The policies could also impact currently serving personnel.

"We have a great military and I want to keep it that way," Trump told ITV.

"And maybe they'd be phenomenal. I think they'd probably would be, but again you have very strict rules and regulations on prescription drugs and all these different things and they blow out of the water."

Trump concludes a three-day state visit to Britain on Wednesday by attending a 75th anniversary commemoration of D-Day in the south England city of Portsmouth.

zak/wdb

ITV