Virgin Money Giving has apologised for a website malfunction which meant last-minute well-wishers were unable to donate to London Marathon runners.
Fundraising pages for thousands of participants are hosted on the site, which crashed for several hours on Sunday.
To stop competitors potentially missing out on extra cash from the site going down, the company offered to boost all charity donations made on Sunday and Monday by 10 per cent.
On the morning of the race, a message on the website read: "Sorry, Virgin Money Giving is unavailable at the moment.
"Thanks for your patience while we work to restore normal service.
"We'll be back up and running as soon as possible."
Millions of pounds are donated to charities and campaigns through the Virgin Money Giving pages and many racers would have expected to be buoyed by race-day pledges.
Some 40,000 runners powered through the streets of the capital in dazzling sunshine after the Duke and Duchess of Cambridge and Prince Harry formally started the event.
Jo Barnett, executive director at Virgin Money Giving, said: "Virgin Money Giving would like to congratulate everyone that took part in the Virgin Money London Marathon today. It was another fantastic celebration of all that is good about London.
"Unfortunately Virgin Money Giving was not available for part of the morning and to say sorry we will be boosting all donations made to charity on April 23 and April 24 by 10 per cent.
"We hope that this will be well received by all the charities that our fundraisers support."