UK Independence Party (Ukip) London mayoral candidate Lawrence Webb, who also stood for the Hornchurch and Upminster parliamentary seat, has defended his use of the word “retard” on Twitter, saying his comments should be taken “with a pinch of salt”.

Mr Webb, 45, was referring to remarks made by David Cameron recently regarding the ceding of power to the European Union (EU) – one of the central issues in Ukip’s manifesto.

On Friday (September 28) Mr Webb, of Harold Wood, wrote on his Twitter account: “David Cameron must be a retard! He just doesn’t get it. The only way to ensure any form of self-determination is to leave [the EU].”

Sharron Peel, chief exec of local disability charity Havering Association for People with Disabilities (Had) said Mr Webb’s use of the word “retard” was inappropriate.

“I think he should know better,” she told the Recorder. “That word offends a lot of people and it takes us back about 50 years. I think disabled people have worked very hard for equality and for someone to be using that word – there’s just no place for it.”

But Mr Webb said he had deliberately used a provocative word to get attention. “It’s difficult to explain why the comments David Cameron is making are false or misleading in 140 characters, so one has to use something provocative to get the attention to draw out the debate,” he said. “The reason we are having this conversation is I used a very provocative term.”

International campaign The R-Word was established by the Special Olympics in 2008 to end the use of “retard” and “retarded” as derogatory words.

The R-Word’s website states: “The R-word hurts because it is exclusive. It’s offensive. It’s derogatory. The R-word is hate speech.”

It adds: “[…] ‘retard’ and ‘retarded’ have been used widely in today’s society to degrade and insult people with intellectual disabilities. Additionally, when ‘retard’ and ‘retarded’ are used as synonyms for ‘dumb’ or ‘stupid’ by people without disabilities, it only reinforces painful stereotypes of people with intellectual disabilities being less valued members of humanity.”

However, Mr Webb insisted he had not intended to insult disabled people. “It was never my intention to cause offence to anyone with a mental disability or their family and friends,” he said. “I think one has to take what’s said on Twitter with a pinch of salt.”

A Downing Street spokesman declined to comment.