The Royal Standard was flying above the landmark building about a mile from the Houses of Parliament where four were killed, signifying that she was in residence.
Earlier in the day, she had conducted an audience with the ambassador of Thailand Pisanu Suvanajata and his wife.
As the Houses of Parliament were in lockdown, palace staff said they would not comment on security matters concerning the royal family, because that is an issue for the police.
The Queen’s son and heir Prince Charles was in Yorkshire, northeastern England, carrying out a variety of visits. He started the day at a farm, which is the home to some small businesses, and was between two appointments, having just left the Army Foundation College in Harrogate, where he had been learning more about the training and education there, when news broke of the incident 220 miles south in London.
His “away-day” tour continued and ended when he met patients, volunteers and staff at the Prince of Wales Hospice, Pontefract. his wife, Camilla, Duchess of Cornwall was at a school in Newmarket, about 70 miles north of London.
Kate has an engagement in London on Thursday, to highlight the importance of mental health in mothers as part of the royals’ Heads Together campaign. Also on Thursday, the Queen is scheduled to open the Metropolitan Police’s new headquarters a short distance from the attack.