An undercover police officer has denied that he set fire to a high street department store while masquerading as an animal rights campaigner, a public inquiry has heard.

Six witnesses have told the undercover policing inquiry that Bob Lambert, a police spy, was involved in an arson attack on a Debenhams that caused damage costing £340,000.

Testifying under oath, Lambert repeatedly maintained that all six were wrong. He also denied claims that he had submitted distorted intelligence reports to his supervisors to conceal his own role in the arson plot.

Lambert is the most controversial police spy to have been questioned so far by the inquiry, which is examining how undercover officers spied on more than 1,000 political groups over more than four decades.

As well as his alleged role in the arson attacks on Debenhams, Lambert deceived four women into sexual relationships while he spent five years undercover infiltrating animal rights activists and anarchists in the 1980s.

He fathered a son with one of the women, known as Jacqui, and then vanished from their lives. She has told the inquiry that her life was “absolutely ruined” after discovering the truth by chance more than two decades after their child was born.