Marathon organisers are not expected to publish the final figure until later in the year, but said they were confident the event had now raised more than £1bn in its near 40-year history.
In a statement, Virgin Money Giving said runners using its platform had so far raised £25.5m and it expected the total to be more than £28m. This would be a new record for the platform, which this time last year had helped to raise a total of £21.8m.
JustGiving said runners fundraising through its site had raised more than £20m this year, although by this time last year it had raised £22.3m.
But it said that the amount raised per fundraiser and the average donation amount had both risen this year. It was unable to say what these figures were or how much they had increased by.
The platform added that it had now helped to raise more than £300m for good causes through the London Marathon since JustGiving was founded 19 years ago.
Jo Barnett, executive director of Virgin Money Giving, said: "For the past 10 years we’ve been breaking fundraising records. I am absolutely delighted that Virgin Money Giving and the Virgin Money London Marathon have done it once again.
"More fundraisers than ever have used Virgin Money Giving to support their charities this year and for the first time their supporters have been able to track them and give directly through the marathon’s official app as well as the usual channels.
"Donations continue to pour in and the runners can rest assured that more of their hard-won donations will reach their selected good causes because of our low fees and not-for-profit model."
Keith Williams, general manager and head of UK at JustGiving, congratulated the runners and those who had donated.
"We are proud to say that JustGiving has now helped 200,000 London Marathon participants raise £300m over the past 19 years, putting world-class digital technology to work and supporting the efforts of every runner taking on the ultimate physical challenge," he said.
The official charity partner for next year’s London Marathon is expected to be announced later this week.