Under Spanish law justifying genocide or inciting racism and xenophobia are crimes which can carry a prison sentence of up to three years.

During his speech at a Barcelona bookstore, Irving, a historian who has written over 20 books, said "there is no proof" dictator Adolf Hitler was aware of what was happening in Nazi Germany’s concentration camps, the Europa Press news agency reported.

But he said "there was no doubt" that the Nazis killed two or three million Jews.

Less than 20 people turned out to hear him speak while about 100 people protested outside the bookstore which was guarded by a strong police presence, the agency said.

In 2006 a court in Austria sentenced Irving to three years in prison after he pleaded guilty to denying the Holocaust but he was released and deported to Britain after serving just one-third of his sentence.

He was arrested in late 2005 on charges stemming from two speeches he gave in Austria in 1989 where he said most of those who died at Nazi concentration camps were not executed, but instead succumbed to diseases like typhus.