According to top secret government papers the assassination plans were approved by the then prime minister, Winston Churchill. BBC Foreign Affairs Correspondent David Loyn investigates.

Churchill approved the plans to kill Hitler

One of the last remaining secrets of the World War II is out, revealed under a policy of open government. Operation Foxley, the plan to kill Hitler, has remained a secret for more than half a century.

BBC News' Sancha Berg on the SOE plots

It began when an agent in Algeria said that he had a project to kill Hitler. His message says, "we are not mad and this is not, repeat not, a joke."

Explosives, bullets or poison?

A 120-page dossier was compiled

By late 1944, a huge dossier had been prepared on Hitler's habits. Contingencies were being considered for poisoning him (by putting a toxin in his tea or impregnating his clothes with a lethal bacteria) or blowing up his car or train, or shooting him on his daily walk towards a teahouse at his mountain retreat.

Pictures of uniforms were also collected which could be used to make disguises and agents were testing chemical and biological weapons.

Debate over assassination

Adolf Hitler: poisoned tea or sniper fire?

The head of the Special Operations Executive, the agency responsible for operations behind enemy lines, was quoted as being "not exactly optimistic or enthusiastic" about the idea of killing Hitler.

But by the end of 1944, the net of people who might be assassinated had been widened to include most of the German leader's senior staff.

The documents suggest that an attack on the Nazi propaganda chief Goebbels was actually authorised.

A secret debate raged in Whitehall over the plan to kill Hitler. The SOE wanted to give it a try but other political and military leader preferred to bomb Germany into submission. They thought that assassinating Hitler would only turn him into a martyr.

Hitler 'more valuable left in place'

The files have been locked away for over 50 years

There was also substantial opposition from those who felt that Hitler was more valuable left in place because of his mistakes in managing the war.

One memo by Lieutenant Colonel RH Thornley, code-named X, said that Hitler's value was equivalent to an "almost unlimited number of first class SOE agents strategically placed inside Germany".

SEO agents, known as "bonzos" were renowned for having few scruples -Churchill called them "the ministry for un-gentlemanly warfare".

'Low methods'

Descriptions of uniforms were compiled

But even among the SOE some questioned whether they should kill Hitler. Lieutenant Colonel RH Thornley expressed concern at the time about the UK resorting to such "low methods" as assassination to win the war.

Some of the proposals were clearly ludicrous, including the idea that Hitler's deputy Rudolf Hess, who was in gaol in the UK, should be hypnotised into killing Hitler.

But by 1945, although a more credible assassin than Hess was prepared to carry out the attack, the agents ran out of time and the war ended before an attempt could be made.

Enemies and allies

Maps were made of Hitler's summer home - for snipers to use

The documents also show that the UK tried to cause confusion by issuing stamps with the head of Himmler on them, to make it look as if he had taken over.

But there are revelations of major embarrassments too, including a dispute with the other main secret service operating in occupied Europe, MI6, over German penetration of a spying network in Holland.

MI6 agents were accused of not passing on their suspicions that the Germans were running the whole of the UK network, so that new agents continued to drop unwittingly into the country.

VE day: time ran out for those planning the assassination

The documents also reveal arguments between the Churchill government and the French General Charles de Gaulle, who was exiled to the UK.

The UK was trying to set up a French resistance network, but he insisted on running it.