London Marathon organisers are concerned a number of elite athletes may miss this weekend's race because of the ongoing travel problems.
The drift of volcanic ash from Iceland has closed British airspace although some of it will reopen today.
Organisers say the race will definitely take place but most of the top-class field are currently stranded across Africa, Europe and the United States.
They say they are looking at all alternatives to get athletes to London.
Among the athletes likely to be hampered are defending champions Sammy Wanjiru from Kenya and Germany's Irina Mikitenko as well as Britain's Mara Yamauchi, who is based in Japan.
It is hoped some of the European-based competitors in the elite races may be able to travel overland to Britain but there could problems for the competitors based further afield.
The travel chaos has been caused following a significant eruption at the Eyjafjallajoekull volcano in Iceland, the second in less than a month.
Marathon organisers added that they are monitoring the air pollution levels in London and will be providing more information to runners later in the week.
At present, they say that analysis of the dust material from the Scottish Environmental Agency indicates that it does not contain material that is seriously harmful to health.