The London Marathon press office confirmed to the Mail & Guardian on Wednesday that four men and two women from South Africa are registered to participate in the event.
At the time of publishing, marathon organisers had received no cancellations from the South African team.
A Cape Town-based travel agency, who asked not to be named and deals mainly with sporting events, confirmed to the M&G on Wednesday that a group of South Africans left for London this week.
The agency said it handled travel bookings for runners who participated in the Boston Marathon as well as those participating in the London Marathon.
"We can confirm that the Boston athletes are all safe and in good shape and that they have not cancelled their participation in the London event," a travel agent said.
According to the London Marathon's website, South African wheelchair and hand cyclist Ernst Van Dyk is scheduled to participate. Van Dyke tweeted on Wednesday morning:
"Have arrived in London. Typical English weather to welcome us. Will be interesting to see how the vibe is at race hotel."
On Tuesday night, Van Dyk tweeted: "Moving on to London today. Yesterday's events are still so surreal. I'm leaving Boston with a heavy heart and great sadness."
Van Dyk, who has won the Boston Marathon nine times, was an eye-witness to the twin bombings which killed three people and injured more than 180 athletes and spectators on Monday.
Two South Africans were injured in the blasts, but the department of international relations and co-operation (Dirco) confirmed to the M&G that they were spectators.
Dirco spokesperson Clayson Monyela said the two sustained minor injuries, were treated at the scene and released immediately thereafter.
Meanwhile, Athletics South Africa (ASA) said it could not confirm how many South Africans were participating in the London Marathon, as athletes had failed to register with the organisation.
Mehlo Hlabangale, from ASA's department of development, said this was the case with the Boston Marathon too.
"Internationally, athletes are supposed to inform their home federations about their plans to participate in events overseas. But South African athletes have not been keeping us up to date, so we can't say who is participating in the marathons (Boston and London) with any certainty," he said.
Athletics South Africa was placed under administration by the South African Sports Confederation and Olympic Committee (Sascoc) last week.