A Cold War railway control centre designed to survive an atom bomb attack has been given listed building status.
The disused Air Raid Precaution Railway Control Centre was constructed in Station Road, Bricket Wood, in 1954 as a defence against Soviet aggression.
In the event of a war, key rail workers from London Euston were to be sent there to keep the trains running.
Historic England has recognised the building as being of "both special architectural and historic interest".
The Grade II listing means it will be added to the National Heritage List and given extra protection under planning law.
The secret control centre, near St Albans in Hertfordshire, measures 36ft (10.9m) by 95ft (28.9m) and was built beside Bricket Wood rail station at a cost of £8,500 - about £230,000 in today's value.
The 2ft (0.6m) exterior, windowless walls are made of reinforced concrete with two steel doors at either end.
It was hidden in a wood, more than 10 miles from the centre of London, the likely epicentre of a missile strike, and was to be one of 25 centres around the country, although only five were completed.
Falling into disuse decades ago, it was at one time used for storage by a building supplies company.
Historic England said it was of historic significance because it is "tangible evidence of the strategic national response to the perceived threat of hostility to British government, security, infrastructure and the civil population during Cold War tensions".
St Albans district councillor Mary Maynard said: "I am pleased that it has now been given listed building protection, so that any modifications can only be made with consent from us."