Peter Ricketts, who was a former top civil servant at the Foreign Office, said on Tuesday that the United Kingdon's invitation to US newly elect US President Donald Trump should be downgraded to an official visit to spare the queen any embarrassment.
More than 1.6 million people have signed a petition to cancel the state visit and thousands took to the streets in protest following Trump's decision to ban refugees and citizens of seven mainly Muslim countries.
Ricketts, a House of Lords member who also served as national security adviser to former prime minister David Cameron and as ambassador to France, called the invitation "premature" in a letter to The Times newspaper.
"It would have been far wiser to wait to see what sort of president he would turn out to be before advising the queen to invite him. Now the queen is put in a very difficult position," he said.
State visits involve Queen Elizabeth II, who is the head of state, and are seen as the highest expression of friendly ties between two countries. Official or working visits are less ceremonial and usually do not include royal engagements.
May announced the state visit during a meeting with Trump at the White House on Friday which was intended to boost trade ties after Britain leaves the EU. "There is no precedent for a US president paying a state visit to this country in their first year," Ricketts said. "The government needs to move fast to protect Her Majesty from the growing controversy."
The Times quoted sources close to Buckingham Palace saying that the state visit was going ahead on May's recommendation, after a Downing Street spokesman appeared to shift the blame away from her entourage.
One source said that royal officials "don't want the queen being dragged into something political".
But former foreign minister William Hague, writing in the Daily Telegraph, played down the concerns.
"A queen who has been asked over the decades to host tyrants such as Presidents Mobuto of Zaire and Ceausescu of Romania is going to take a brash billionaire from New York effortlessly in her stride," he wrote.