The Cardiff Half Marathon, due to take place in October of this year, has been officially postponed because of the coronavirus pandemic.
Around 25,000 people usually take part in the 13.1 mile event around the capital city, with around 100,000 spectators also lining the streets.
The news was confirmed by Run 4 Wales, the company that helps organise the mass-participation sporting events in the country.
There are concerns that the economy in the capital could miss out on £5 million as a result.
The Chief Executive of Run 4 Wales, Matt Newman, said that organisers had been monitoring the situation closely but had made the 'difficult' decision with safety of the public in mind.
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," he continued.
"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."
There is some good news though as the events have not been cancelled completely, with new dates in March and April of next year set.
Having spent a significant amount of time talking with our sponsors, charity partners, Local Government and other key stakeholders, we have no choice but to postpone this year’s Cardiff Half Marathon and reschedule the event on 28 March, 2021. The Newport Wales Marathon will also be postponed to 18 April, 2021.
"We have decided to communicate this decision now to provide clarity to everyone. The Cardiff Half Marathon has become one of Wales’ most iconic sporting events which provides an opportunity for club runners and the public to run alongside some of the world’s leading endurance runners," Mr Newman continued.
"As well as raising the profile of the Welsh capital to an international audience, and driving more than £5m of economic value to the region, the event highlights the benefits of running to improve physical and mental health.
"We are working with a group of leading events organisations in Wales toestablish guidelines for the safe return of events."
The Cardiff Half Marathon has been running for 17 years and has become Wales's largest multi-charity fundraising event and sits within UK Top10 Events for Sports Fundraising.
This year's Cardiff event sold out in record time and participants who had purchased a ticket for any of the cancelled runs should have been contacted regarding options available to them.
The organisers of the event have said there will be two events in Cardiff in 2021, with the postponed taking place in March as well as the usual October half marathon still going ahead.