Last-minute donors who have tried to log on greeted by a message saying the service is "currently unavailable"
Thousands of people are running - but the London Marathon's official fundraising page isn't.
Virgin Money Giving has crashed on the morning of the race - with last-minute donors who have tried to log on greeted by a message saying the service is "currently unavailable".
The website also suffered a short outage on Saturday night because "lots of donations were being made" - but the organisation had insisted the technical issues were resolved.
Thousands of runners have relied on the Virgin Money Giving website to raise cash for their chosen charities, and many would have been hoping to benefit from a race-day surge in pledges.
"Virgin Money Giving are aware of the performance issue and the IT team are looking into it," a spokeswoman said.
Dozens of exasperated donors have been asking the website for updates on Twitter.
One runner wrote: "At prime time for donations, you have really let me down, I am sorry to say. Hope I haven't lost too many donations as a result."
A record number of competitors are tackling this year's marathon, with the Duke and Duchess of Cambridge - as well as Prince Harry - set to cheer the 40,000 runners as they tackle the 26.2-mile course through the capital.
Hundreds of police have been deployed to keep the runners safe, in addition to the estimated 760,000 spectators who are expected to line the streets.