Condoms full of purple flour were flung at Tony Blair as he faced MPs in the House of Commons on the day in 2004.
The first fell at his feet and the other exploded off his shoulder, sending flour wafting through the hallowed chamber.
The incident forced Prime Minister's Questions to be abandoned and MPs were asked to swiftly vacate the area.
The projectiles were thrown from the gallery reserved for VIPs and guests of MPs or peers. Two men were immediately arrested.
Campaign group Fathers 4 Justice claimed responsibility, saying the stunt was designed to draw attention to their fight for better access for divorced fathers to their children.
Despite the innocuous nature of the attack, the implications were extremely serious - it came just weeks after the public gallery was glassed off to protect MPs from lobbed phials of deadly toxins such as anthrax or ricin.
Indeed, after MPs had evacuated the chamber, police officers in chemical and biological suits checked whether the purple haze was dangerous.
It soon emerged that the two perpetrators gained access to the open-faced VIP gallery as guests of Labour peer Baroness Golding, and she offered "unreserved apologies" to the Speaker, MPs and fellow peers.
The flour bombs were thrown by Ron Davis (above, centre), from Worthing, West Sussex, who was campaigning for access to his two children. He had recently raised his case with the Prime Minister during a radio station phone-in but declared himself dissatisfied with Blair's "bland" response.
His accomplice, Guy Harrison (above, right), also from West Sussex, held up a Fathers 4 Justice poster during the attack.
Davis and Harrison were both convicted of disorderly behaviour and fined £500 and £600 respectively.
[May 2, 1997: Labour win general election by a landslide to end 18 years of Conservative rule]
- Founded in 2002 by Matt O'Connor, Fathers 4 Justice aims to gain public and parliamentary support for changes in UK legislation on fathers' rights, mainly using stunts and protests often conducted in costume.
- The group came to national prominence in October 2003 when two of its members - Eddie Gorecki and Jonathan Stanesby - scaled the Royal Courts of Justice, dressed respectively as Batman and Robin.
- Stanesby went on to make a citizen's arrest of Margaret Hodge, then minister for children. The MP was handcuffed to Stanesby for about 20 minutes after being ambushed at a Law Society conference in Manchester. She was eventually free with bolt cutters. Stanesby and an accomplice were later cleared by a jury of charges of false imprisonment.
- Later in 2004, Fathers 4 Justice campaigner Jason Hatch spent five hours on a ledge at Buckingham Palace dressed as Batman, while the Foreign Office (below), Blackpool's Big One rollercoaster and a gantry on the M25 would all be targeted by costumed campaigners.
- In 2006, the Sun newspaper claimed that Fathers 4 Justice planned to kidnap Tony Blair's son, Leo, "for a few hours as a symbolic gesture" - but police say the plot never got beyond the "chattering stage".
- In 2013, the group began to target works of art. Paul Manning glued a picture of his 11-year-old son to John Constable's 1821 painting The Hay Wain in the National Gallery in London, and Tim Haries spray-painted the word 'Help' on a portrait of the Queen at Westminster Abbey. After being sentenced to six months in custody, Haries claimed he was a "political prisoner".
- The group's image was tarnished by an ugly and protracted dispute between the founder, Matt O'Connor, and Caroline Nokes, his local MP. Nokes sat on the committee which drew up the Children and Families Bill, and O'Connor was reportedly upset that his group was only invited to tender written rather than oral evidence. Threats made on Twitter were reported to the police by Nokes, but it was O'Connor who took the MP to court in 2014, claiming harassment. The judge threw out the case, citing "a total absence of evidence".