Quote from her wikipedia page:
The race however is remembered more for a notorious moment towards the end when Radcliffe, feeling hindered by bowel discomfort and in need for a toilet break, stopped and defecated on the side of the road in view of the crowd and TV cameras which broadcast the incident live. After the race she apologised to viewers and explained what happened, “I was losing time because I was having stomach cramps and I thought ‘I just need to go and I’ll be fine’. I didn’t really want to resort to that in front of hundreds of thousands of people. Basically I needed to go. I started feeling it between 15 and 16 miles (26 km) and probably carried on too long before stopping. I must have eaten too much beforehand“. In November 2006, the incident was voted top running moment in history in the UK from a choice of ten ‘unforgettable moments’.