The 26,000 runners due to take part in this year's Cardiff Half Marathon have been told that it is being postponed until 2021 because of the coronavirus pandemic.
Organiser Run 4 Wales says holding it in just four months' time "really isn't feasible" and the event and a new date has been set.
It will now take place on March 28, 2021.
The Newport Wales Marathon - already postponed from April to October - has been pushed back again to a new date of 18 April 2021.
Runners who have entered the October run in Cardiff will have their places automatically transferred to March, but organisers say they are not able to offer refunds.
Organisers say: "Sadly, we are not in a position to offer refunds, however we are offering a number of reasonable alternative options to runners, which have been outlined to all participants via email.
"We hope that these options will suit the vast majority of runners who will understand our position given current circumstances."
Chief executive of Run 4 Wales Matt Newman said: "The news is disappointing and will be to all the runners that had signed up to this year's event however I think frankly that the scale of any event like the Cardiff half marathon with 26,000 people signed up... the event simply can't be held safely in October this year."
Since its foundation in 2003, the event has become the UK's third biggest race after the London Marathon and the Great North Run.
Last year 27,500 runners and 100,000 spectators attended the race.
Mr Newman explained the thinking behind the postponements, saying: "We have been closely monitoring the situation surrounding the COVID-19 outbreak and the guidance issued by the Government and it is now clear that it will not be possible to stage events of this scale in the autumn.
"We would expect the Welsh Government to make further announcements in the coming months on a gradual return to a full sporting calendar. Although lockdown restrictions have recently been eased in Wales, we understand there is no possibility of 27,500 participants and 100,000 spectators gathering safely in Cardiff on 4 October and thousands more three weeks later in Newport on 25 October.
“We are also conscious of the inter-dependency on various service providers for events of this size, including the availability of medical support services, volunteers, emergency services, transport and accommodation."
The Cardiff Half had been due to take place on the same day as the re-arranged London Marathon.
The 2021 race is scheduled for the first weekend in October as usual (Sunday 3). Entries for the October 2021 race will be going on sale soon, but only a limited number will be available.
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