Measuring 10 metres long, 2.5m wide and 2m high, it is believed to have been built around 1940, and has laid unnoticed under Tariff Road in north London until last month when plans were being drawn-up for a Thames Water sewer upgrade scheme.
It is buried less than a metre below what is now the driveway for an industrial area.
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It can only be accessed via manholes and doesn't show up on any mapping so the new sewer pipes had been due to pass directly through it. They have now been changed to preserve the important piece of history.
Mike Lang Hall, archaeological adviser to Thames Water contractors Optimise, said: "It wouldn't have been right to demolish the shelter as it could have saved many lives during the war.
"Our project must go ahead to protect nearby properties from flooding but now we're doing it in a way so as not to disturb such a precious piece of our heritage. There's no record of ownership but we suspect it was built to serve the local community and possibly workers at the nearby Triumph factory."
(CD/JP)
