Russian distance runner Liliya Shobukhova is free to compete again after her doping ban was ended early for assisting other investigations.
Shobukhova was also stripped of Chicago Marathon wins in 2009, 2010 and 2011.
Her 38-month ban has been cut to 31 after offering "substantial assistance" to the World Anti-Doping Agency (Wada).
"The information and documentation provided has been of substantial value in uncovering and investigating anti-doping rule violations committed by other individuals, including athlete support personnel," a Wada statement read.
Shobukhova was handed an initial two-year ban in April 2014 - backdated to January 2013 - but a further 14-month extension was applied by athletics' world governing body, the IAAF, earlier this month following a ruling by the Court of Arbitration for Sport.
That would have seen Shobukhova suspended until March 2016 before Wada's intervention.
London Marathon organisers intend to take legal action "to recover the prize and appearance money" paid to Shobukhova in both 2010 and 2011, when she finished as runner-up.
"The London Marathon's zero-tolerance policy towards doping is unaltered," chief executive Nick Bitel said on Monday.
"Shobukhova is still banned for life from taking part in the London Marathon and in any of the five other marathons that make up the World Marathon Majors.
"We believe there should be no place in our sport for people who have cheated, no matter how much assistance they give after they have been caught."