Fred Phelps and his daughter Shirley Phelps-Roper had planned to fly into Britain to protest against a play being put on by a gay youth group.
The pair are members of the church, based in Kansas, which has been described as "the most evil religious sect in the world".
Mr Phelps, 79, and his daughter Ms Phelps-Roper, 51, had targeted a performance of The Laramie Project, about the death of an American man killed for being homosexual.
They had announced that they would picket the play, which is due to be held at a school arts centre in Basingstoke, Hampshire, on Friday.
But the pair will now be stopped by immigration officials if they arrive and placed straight on a flight back to the US.
The ban comes after Dutch MP Geert Wilders was barred from the UK last week for his extremist views on Islam.
A UK Border Agency spokesman said: "The Home Secretary has excluded both Fred Phelps and his daughter Shirley Phelps-Roper from the UK.
"Both these individuals have engaged in unacceptable behaviour by inciting hatred against a number of communities. The Government has made it clear it opposes extremism in all its forms.
"We will continue to stop those who want to spread extremism, hatred and violent messages in our communities from coming to our country. That was the driving force behind the tighter rules on exclusion for unacceptable behaviour that the Home Secretary announced on 28th October last year.
"The exclusions policy is targeted at all those who seek to stir up tension and provoke others to violence regardless of their origins and beliefs."
Members of the Westboro Baptist Church, mostly Phelps family relatives, regularly picket funerals of US war heroes, claiming their deaths are punishment for America's tolerance of gays.
Fred Phelps has also described the Prophet Mohammed as a "demon-possessed whoremonger" and once said Catholics were part of the "church of the holy paedophiles".
