The UN agency which organises Gaza's marathon has cancelled the event, blaming the refusal of the territory's governing Islamist Hamas movement to allow women to run.
Hamas said the marathon could go ahead if "local traditions" were respected.
"We regret this decision to cancel the marathon but we don't want men and women running together," Abdessalam Siyyam, cabinet secretary of the Hamas government, told AFP news agency.
"We did not tell Unrwa to cancel the marathon and we haven't prevented it, but we laid down some conditions: We don't want women and men mixing in the same place," he added.
In the past there have been attacks on the UN's summer camps for children in Gaza after complaints that boys were allowed to mix with girls, the BBC's Jon Donnison reports.
The marathon was due to be part of the UN's fundraising efforts in order to run those camps, our correspondent adds.