Vandals have damaged the glass splash panel being trialled as a wave defence barrier on Sidmouth Esplanade.
East Devon District Council (EEDC) has confirmed that deliberate criminal damage was the cause of the widespread cracks across the panel.
CCTV footage is being inspected and police are investigating the incident, which took place on the night of Tuesday, March 10.
A spokesman said: 'Our StreetScene team has inspected the panel and although the weather was unsettled yesterday evening, they have concluded that this was not the cause of the damage.
'They have confirmed that although the sea facing side is still intact, deliberate criminal damage has clearly been inflicted on the glass's landward facing surface - possibly with a hammer or similar heavy instrument.
'The trial of the panel is due to finish at the end of April and so far the glass appeared to be holding up well, having weathered the impact of three major storms.
'In light of its robustness to date it is therefore disappointing and immensely frustrating that it has succumbed to this malicious and destructive act.'
EDDC said this was not the first time the panel had been vandalised; three chips on the land-facing side that could not have been caused by shingle damage were spotted recently.
The repeated attacks cast doubt on the feasibility of having a transparent wave barrier instead of a solid wall.
The spokesman said: 'If a glass panel is going to be subject to repeated damage by vandals, then it will not be sustainable.
'We will now have to consider very carefully, whether the use of glass panels to minimise the visual impact of the splash defence is a material option that the council can take forward.'
Ward member, Cllr Denise Bickley, said: 'Potentially having to withdraw this idea due to vandalism is such a pity and unfortunately will say something about the town rather than about the glass itself.
'I sincerely hope the CCTV footage can identify the perpetrators of this act and we should all call it what it was - criminal damage to property, and not something to be bragged about.'
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