The Great South Run has been cancelled due to a forecast for "high winds and rain" - as well as a risk of "high tide levels and "potential debris".
More than 20,000 runners were expected to take part in Sunday's event along the coast in Southsea, Portsmouth.
Organisers said they had been monitoring the weather but it hadn't improved enough for the event to be staged safely.
It comes as Storm Ashley is expected to bring winds up to 80mph to parts of the UK on Sunday.
A statement from the run's organisers said: "The forecasted winds and gusts compromise our event infrastructure including our medical facilities at the finish and around the course.
"This combined with high tide levels, potential debris, and the exposure along the seafront mean that we can't guarantee the safe delivery of the event.
"We haven't taken this decision lightly and have been liaising closely with the Met Office, medical & safety teams, local authorities and key stakeholders throughout the week to make the best-informed decision.
"We know how disappointing this is and we hope you can understand why this decision has been made."
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