The fountains in Trafalgar Square will turn green today as part of a festival of fun to mark St Patrick's Day.
Thousands are set to take to the streets for a parade and festival in central London, marking the contribution of the Irish community.
London's fifth annual St Patrick's Day celebration will include a parade featuring more than 4,000 musicians, dancers, Irish bands and floats.
Prime Minister Tony Blair said: "This wonderful event has now become a fixture in the city's calendar, reflecting the enormous contribution of the Irish community in London over centuries, as well as their current and continuing achievements."
The Taoiseach Mr Ahern said: "Britain is home to a very strong and vibrant Irish community, over a third of whom live in the greater London area. I know that they will be joined by tens of thousands of other Londoners in celebrating our National Day reflecting the strength and pride of the community in their heritage and culture."
PA