By Guto Thomas BBC Wales political reporter

Documents about the plan have only ust been released

Newly declassified documents, released from the national archieves at the Public Record Office in Kew, reveal a plan, called "Mr Mills' Circus", to prepare for a German invasion.

In January 1941, the double agents were regarded as a security risk, and it was feared that they would pass on information to the Germans if captured.

It was decided that the agents were "too dangerous to leave at large at a time of invasion" and that north Wales was the safest place to hide them.

Invasion fears were running high at the time after the forced evacuation of the British Expeditionary Force from Dunkirk in June 1940.

Although the British had captured and were controlling all German agents in the UK, there were concerns they could switch sides in the event of invasion.

Two of these agents were Welsh - one known as Snow, whose real name was Arthur Owens.

The other was known to MI5 as GW. Gwilym Williams was in fact a retired policeman from Swansea.

The Eagles Hotel in Llanrwst could have housed double agents

In March 1941, a letter was sent to MI5's contact in north Wales to make arrangements to "look after the safe custody of these individuals" in the event of an invasion.

Captain PES Finney, who was based in Rhos-on-Sea, was told the double agents would travel by car, under armed escort, and would need accommodation.

As a result, arrangements were made for the double agents, their families and minders to stay at hotels including the Eagles hotel in Llanrwst, the Swallow Falls Hotel in Betws y Coed and the White Heather Hotel and the Evans Hotel in Llandudno.

Within a few weeks Captain Finney told London he had "completed arrangements of the animals, their young, and their keepers ..."

However, the plans were scrapped two years later, as the tide of war changed and the possibility of invasion receded.

When the scheme was scrapped in late 1943, all papers relating to the operation as well as materials including handcuffs, petrol vouchers, cash, pistols and ammunition had to be returned to headquarters.

The codename was Mr Mills' Circus because the senior MI5 officer in charge was CB Mills, whose family ran the Cyril Bertram Mills Circus.