Spanish ensign captured at the Battle of Trafalgar, belonging to the San Ildefonso. The flag had been hung in the crossing of St Paul's Cathedral during Admiral Nelson's funeral service on 9 January 1806.
San Ildefonso was captured after only an hour of action in the battle. The ship came under heavy fire from the Revenge and Defence, together shooting away her mizzen and main masts. With 34 dead and 126 wounded, she surrendered to the Defence.
San Ildefonso was one of only four prizes captured by the British at the Battle of Trafalgar, along with the Bahama, Swiftsure and San Juan Nepomuceno, surviving the storm that followed the battle. She was used as a store ship and broken up in 1816.\
Source: http://www.rmg.co.uk/content/identifying-san-ildefonso-flag