Mary Whitehouse sent letters from teachers worried about the impact "video nasties" were having on their pupils to the highest echelons of the government as part of her long fight against obscenity.

As the voice of conservative Britain, Mrs Whitehouse spent decades campaigning against what she perceived as the increasing liberalisation of society and the media and a general decline in moral standards.

Her years of activism from the 1960s onwards made her both a figure of fun and a thorn in the side of the government and the BBC, which she repeatedly attacked over her belief that it excessively portrayed bad language, violence and sex.

But according to files released by the National Archives in Kew, west London, during her fight in the early 1980s against overly-violent films - so-called "video nasties" - she appeared to find an ally in the then-Prime Minister, Margaret Thatcher.

In a reply to Mrs Whitehouse after she phoned the Prime Minister's office in early 1983 urging fresh action on obscenity legislation, Mrs Thatcher wrote: "I fully understand, and indeed share, your deep concern about the decline in moral standards in this country.