London Marathon organisers have delayed a decision on when the 2020 event can be held until 7 August.
The 40th edition of the race was originally scheduled for 26 April but was pushed back to 4 October at the start of the coronavirus outbreak.
An announcement on it taking place on that date was expected on Tuesday.
However, event director Hugh Brasher, said "further consultation with local NHS Trusts, the emergency services and local authorities" is needed.
In a letter to participants, he added: "I realise that this uncertainty is far from ideal and I would like to thank you for your patience and understanding in these very challenging circumstances.
"We know how important the London Marathon is to you, to charities and in showing the world the wonderful spirit of London, of Great Britain and of our running community.
"So please bear with us while we finish the extensive work we have been doing to try to enable us to run together, safely."