Ukulele-playing star George Formby drove the 1939 Mercury Eight Series 99A Estate as part of the Entertainments National Service Association during a 53-day tour in the summer of 1943.
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However, experts had thought the deterioration was slow moving and that the east wing was affected similarly.
A spokesperson for the charity said until the storm of 23 February, there were no signs of any major structural problems.
The incident occurred just days before remedial works were due to start at the historic site
The damage occured to the east wing wall of the castle, which was built by Henry VIII
Engineers are now assessing the damage to the 573-year-old building and are planning remedial works.
Hurst Castle through the centuries
When Henry VIII built Hurst Castle it was considered one of the most advanced fortresses of its time