This is so abnormal that you just cannot cater for it. F. Castell, Prison Officers' Association

They coshed one of the two patrolling warders on duty and tied him up before opening Wilson's cell door and freeing him.

It is still not known how they got hold of the keys to Wilson's cell. Winson Green is a maximum security prison, and only one member of staff holds the keys to open cells at night.

At a news conference, the secretary of the Prison Officers' Association, F. Castell, said security arrangements at the prison should be enough to meet all normal requirements.

"But," he said, "today's happenings are abnormal. It seems likely that somehow or other a master key has been obtained which allowed these people to effect a simple entry to the prison after scaling the wall.

"This is so abnormal that you just cannot cater for it."

The Home Secretary, Henry Brooke, was on an official trip to the Channel Islands, but returned to London immediately.

He said he was "seriously concerned" by the escape, and ordered an inquiry to begin straight away.

Wilson is described as one of the masterminds behind the robbery last year. In the biggest heist of its kind, over £2.5m was stolen from a Royal Mail train.

Most of the money has never been recovered, and Wilson is believed to be the robber who knows where the missing money stolen in the raid is hidden.

He is reported to have told the police who arrested him two weeks after the robbery: "I don't see how you can make it stick without the poppy [money], and you won't find that."

He served just four months of his 30-year jail sentence before his escape.

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