Parkrun will not return on 5 June in England because many venues are reluctant to reopen.
Organisers said that while more than 250 events have landowner permission to return, this falls far short of the number needed to prevent overwhelming those that have been given the go-ahead.
"With more than three million registered parkrunners across England, and around 300,000 people taking part on a typical weekend, opening a small subset of parkruns is not viable," they said.
It said talks are ongoing with landowners "withholding permission" to seek clarification on whether permission would be granted for a later date.
A decision would be made on 11 June on whether to resume all 5k events across England on 26 June, it added.
Nick Pearson, parkrun global chief executive, said: "We will continue to work hard with landowners over the coming weeks to secure the permissions needed to return at the end of June. We are incredibly grateful to all of the landowners who have granted events permission to return so far, and for the groundswell of support for parkrun over recent days.
"Although it is disappointing not to be able to return on 5 June, we are still optimistic that we can return parkrun events to nearly 600 communities across England very soon. We will do everything we can to ensure events restart on 26 June."
Parkrun said the three-week delay comes as many permission requests "got caught up in unnecessary red tape and lengthy internal bureaucracy at a local level" amid a "changing lockdown landscape" despite widespread support from the government and MPs.
Culture Secretary Oliver Dowden said ministers are to write to local authorities and landowners warning them that they must allow the restart of parkrun events.
He, along with London mayor Sadiq Khan and World Athletics president Lord Coe, have backed the return of the event.