PM's Joke Falls Flat.

David Cameron was accused by campaigners today of breaking the law by cracking an ''appalling'' joke comparing Commons Speaker John Bercow to one of the Seven Dwarfs.

A charity which helps people with primordial dwarfism and other forms of restricted growth said the comments were a form of harassment - and called for talks with the Prime Minister.

In a dig at Mr Bercow - believed to be 5ft 6in tall - Mr Cameron said that health minister Simon Burns's driver had accidentally hit the Speaker's car while reversing.

When Mr Bercow said he was ''not happy'' about the incident, Mr Cameron quipped to journalists at a Westminster lunch yesterday, Mr Burns retorted: ''So which one are you?''.

In June, Mr Burns publicly apologised after calling the Speaker a ''stupid, sanctimonious dwarf'' in the Commons.

He said he was sorry for any offence he had caused after the insult was branded ''derogatory and deeply offensive'' by the Walking with Giants Foundation (WWGF).

The charity said it considered all such jokes as a form of harassment, contrary to equalities law, that enhanced negative stereotypes.

Mr Cameron's spokewoman said he had made ''light-hearted comments not intended to cause offence''.

But John Connerty, WWGF co-founder and charity secretary, launched a fierce condemnation of the Prime Minister's decision to ''glorify'' the previous incident.

''Once again we have an influential figure, if not the most influential person in the country, making distasteful references about a person's lack of height,'' he said.

''It was bad enough having a minister of health make similar distasteful comments but to have the Prime Minister himself glorify the comments in a joke is totally unacceptable and downright appalling.

''It appears that certain Government officials are above the law when making derogatory references and associations to a disability.

''It would be nice to know if Mr Cameron and his Cabinet have a diversity policy that they can refer to. If they do, I think they should read it so that they can understand what effect derogatory comments and jokes like these have on a disabled person's mental well-being.

''The Prime Minister should take time out to meet, listen and speak to individuals affected by real cases of extreme short stature to understand how comments and jokes like these affect them and their families.

''However, I expect that as usual an apology will be rendered instead. Then I wonder how long before we have to wait until another apology is issued in response to the next derogatory comment about a person's lack of height is made by a member of the Government.''