Liberty, the human rights group, will defend anyone prosecuted for failing to return their census forms or for failing to declare their ethnicity on the forms.

A spokesman for the group said that it should any defence would be based on human rights legislation covering the right to privacy.

The Office for National Statistics has bowed to pressure and extended the deadline beyond the original deadline.

The census helpline has received one million calls in the last ten days and it has emerged that thousands of homes are still waiting for their forms to be delivered.