The Office for National Statistics has moved to allay fears that damaged census forms could cause a problem for scanning machines.

The machines have been installed to read 33 million forms due to be completed on Sunday April 29, and have been designed to read all styles of handwriting. Nevertheless, 1200 people have been employed to analyse seriously damaged forms.

The announcement comes after thousands of calls were made to a census helpline, leaving many with a two minute recorded message before being cut off. In response to demand the number of phone lines has been increased from 400 to 870.