Wartime members of the Home Guard - famously portrayed in the TV comedy series Dad's Army - reacted with anger today to a new documentary which they claim ''slurs'' Britain's last line of defence.

Viewers of Channel 4's Secret History: Dad's Army, to be screened next month, will be told some volunteers were so incompetent they claimed lives of those they were supposed to protect.

Sir James Spicer, a former Tory MP, who joined the Home Guard ''illegally'' at age 15, denounced the allegations as a ''grave slur''.

''I am dismayed that more than half-a-century on from the formation of the Home Guard, a so-called expert should dredge up facts and figures to suggest that the Home Guard was a shower,'' said Sir James, who retired as MP for West Dorset at the election.

''This is a grave slur and part of the current trend of denigrating the good things of the past.''

Sir James said he tried to join the Home Guard in 1940 at the age of 14, but failed. A year later he did join, but had to pretend he was 16, the minimum age.

He said: ''I was proud to be a member of a commando group in the Middlesex Battalion. We were efficient, fit, reasonably well-equipped and above all had a hard-core of those who had seen front-line service in the First World War and who were then only in their 40s.

''I am absolutely certain that we would have given a good account of ourselves in combat and we certainly played a major part in guarding vulnerable installations. But, alas, it is now part of life to denigrate everything that goes on.''

Former Tory MP Winston Churchill, grandson of Britain's wartime leader, said from Bahrain: ''It is very easy to poke fun at the Home Guard, especially when the closest any of the people involved in the production have ever been to it is to have watched Dad's Army.''

Mr Churchill added: ''I have nothing but contempt, as do the majority of British people, for those who cast such cheap aspersions on very honourable people who played their part in the defence of our country.''

Veteran Les Dinning, organising a memorial to the Desert Rats in Thetford Forest, Norfolk, spent a year in the Home Guard when he was a teenager before joining the Army. He said: ''I think the criticism is very unfair. Of course, many members were untrained and either very old or very young because they were the only ones left. But in the circumstances they did an excellent job.''

The documentary claims 50 citizens mistakenly died at the hands of the volunteers during the war.

Some deaths were caused by an inability to use rifles and guns provided. Others were caused by over-enthusiastic recruits.

The programme says a 15-year-old boy was shot dead in September 1940 as he cycled to an aircraft factory to watch planes being tested. He had not heard an order to stop because he was deaf.

In another incident, members of the Home Guard nearly killed top fighter pilot Flight Lieutenant James Nicolson, VC, when they suspected he was a German spy after he bailed out of his plane.

The documentary, which aims to tell the real story behind the ''cosy myth'' of the Home Guard, also refers to acts of bravery and heroism by members of the volunteer force. It will be screened on Monday, June 8 at 9pm.

A Channel 4 spokesman said: ''The programme is a full picture of the Home Guard and gives full weight to their undoubted bravery and to elements like the underground resistance unit.''