The Queen visited Albert Embankment on Tuesday to unveil a plaque and cut a cake to celebrate the 70th anniversary of the treaty that established the International Maritime Organisation (IMO).
The IMO is the United Nations agency with responsibility for the safety and security of shipping and the prevention of pollution by ships.
The IMO headquarters building, located on Albert Embankment, was opened by the Queen in 1983.
35 years later, the Queen returned to the building on Tuesday for the IMO's 70th anniversary celebrations.
"We are celebrating 70 years of achievement, in which the truly vital industry of shipping has become safer, cleaner and greener, thanks to the work of IMO," said secretary-general Kitack Lim.
"We are also looking ahead to the exciting new challenges on the horizon.
"Billions of ordinary people, all over the world, rely on shipping every day of their lives – even if they don't realise it or understand it.
"It is our role to ensure shipping can continue to make this vital contribution to global well-being," Mr Lim added.
The Convention on the International Maritime Organization (IMO) was adopted on 6 March 1948 at the United Nations Maritime Conference held in Geneva, Switzerland.
The convention entered into force 10 years later, on 17 March 1958, when the 21st state ratified the treaty. IMO's first meeting was held on 6 January 1959 at Church House in London.