LONDON: Barack Obama may be the world's most powerful man but Queen Elizabeth II forced him to slash by half the number of choppers his Secret Service can land on the Windsor Castle lawns when the US president came over for dinner with the 90-year-old monarch.The Queen called Obama's bevy of choppers "over the top", meaning the Secret Service had to rethink their plans to land about six aircraft in the 300-year-old gardens of her main residence."It was a write-off and the Queen was not amused," 'Daily Express' quoted a royal source as saying."Her Majesty refused to back down and said, 'three helicopters only'. The Secret Service had to go away and think about their plan. The President's officials were told that the Queen regarded Windsor Castle as her family home and the most important of all royal residences," the report said."She rarely imposes her will but when she does people listen - it just took the US Secret Service agents a little time to realise that," the source said.The Queen, whose 90th birthday was celebrated recently, said there was no chance Obama's extensive security backup would be accompanying him to Windsor Castle.Her insistence came after Obama's helicopters damaged the grass when half a dozen of them landed during his last visit in 2011.The engines' heat scorched the grounds and the wheels left divots in one of the UK's oldest lawns, planted during the reign of Queen Anne in the early 1700s.The Queen said only three helicopters, including the president's personal aircraft Marine One, could land on the lawn when Obama, 54, and his wife, Michelle, 52, came for dinner with the monarch and Prince Philip to celebrate her 90th birthday, the report said.An insider said US aides refused to change their plans on security grounds but came around to the Queen's wishes eventually."They said they needed to be within reach of the president at all times. They also wanted guards posted inside the dining hall when he had lunch with the Queen," the insider said." But Her Majesty refused to back down and said, 'three helicopters only'. Eventually the President's aides accepted her wishes," the report said.Obama visited the UK as part of his ongoing three-nation tour which also included Saudi Arabia and Germany.