It is the only surviving Anglo-Saxon door in this country, dating back to the time of Edward the Confessor, the Abbey's founder, who was born 1,000 years ago this year.
The exact year of the felling of the door's wood cannot be determined but research paid for by English Heritage (EH) has shown that it is likely to have taken place in the period 1032-64. The door was probably manufactured in the 1050s.
The door - which is an access point to the abbey's vestibule - is made of five vertical oak planks, held together with three horizontal battens, or 'ledges', and iron straps.
The construction of the Westminster door is unique and shows that it was intended to communicate between two spaces of equal importance.
EH chief executive Simon Thurley said: 'We are delighted to have been able to fund and carry out this important investigative work on Westminster Abbey's historic medieval timberwork in partnership with the Dean and Chapter.
'It is incredible to think that when the door was made, the Norman Conquest had not yet happened and William of Normandy was still a young man of about 20 years old.
'William was later crowned King of England on Christmas Day 1066, just a hundred feet away from the door.
'The discovery has deepened our understanding of the Abbey and its history in the year we celebrate the 1,000th anniversary of its founder's birth,' Thurley added.