A plan to bring America's biggest museum to the Olympic Park is still not a done deal, Boris Johnson has admitted.
The mayor is in negotiations with the Smithsonian Institution about opening its first overseas 'outpost'.
He had a further meeting with officials at the Smithsonian Air and Space Museum during a visit to Washington.
The mayor was shown the world's very first aeroplane, flown by the pioneering Wright Brothers, and spacecraft from the Apollo missions.
He hopes the Smithsonian will open a museum in the proposed Olympicopolis cultural quarter next to the Aquatics Centre.
But the mayor conceded the negotiations were at an early stage and it could be 2021 before the Stratford museum opens.