Home Secretary Theresa May was formally appointed Britain’s new Prime Minister at Buckingham Palace by the Queen on Wednesday.
Earlier in the day, the Queen accepted David Cameron's resignation.
A statement said Queen Elizabeth II has "graciously accepted" Cameron's resignation as Prime Minister and first Lord of the Treasury.
Before leaving Downing Street, Cameron said being Prime Minister had been "the greatest honour" of his life.
He added that Britain was "much stronger" than when he took over.