The Marchioness disaster occurred on the River Thames in London in the early hours of 20 August 1989. The pleasure boat Marchioness sank after being run down by the dredger Bowbelle, near Cannon Street Railway Bridge. There were 131 people on the Marchioness. Some were members of the crew, some were catering staff and others were guests at a private birthday party. Fifty-one of them drowned. The Marchioness was built in 1923 and in 1940 was one of the little ships of Dunkirk.

The party was organised by photographer agent Jonathan Phang to celebrate the 26th birthday of Antonio de Vasconcellos, who worked in a merchant bank. The pair were good friends and business partners in a photographic agency. Of Portuguese family background, Vasconcellos had studied Economics at Cambridge University.

Phang organised a three-part celebration: an eight-person dinner in a flat on Meard Street (only two of the diners survived); a birthday cake and champagne celebration for a group of 30 at the flat; and the party on the Marchioness. Many of those at the party were also in their twenties; some were former student friends and others worked in the fashion industry. The dead included Francesca Dallaglio, older sister of future England national rugby union captain Lawrence Dallaglio, and the skipper of the Marchioness, Stephen Faldo, father of reality TV star Jeff Brazier.