An east London council is taking legal action to try to force the organisers of the 2012 Olympic Games to re-route the marathon back to the East End.
London 2012 said operational issues made a finish at the Olympic site in Stratford impossible, as had originally been planned.
Tower Hamlets council said it would now seek a judicial review to have the decision quashed.
London 2012 said the decision to change the route was "fair and legal".
The route, which is now mostly in central London, replaces an original planned course which took in much of Tower Hamlets and the East End.
The new route, announced in September, passes Buckingham Palace, Birdcage Walk, St Paul's Cathedral and the Houses of Parliament.
Tower Hamlets had described the change of the marathon route as a travesty and Rushanara Ali, Labour MP for Bethnal Green and Bow, claimed it was because organisers were "ashamed" of the East End - an accusation denied by London 2012.
Mayor of Tower Hamlets Lutfur Rahman said: "We have not taken the decision to seek a judicial review lightly.
"However we feel that there is no alternative option open to us.
"We believe that Locog (the London organising committee for the Olympic Games) has acted unlawfully in re-routing the Olympic marathon and failed to adhere to the spirit of the original bid which sought to showcase East London, including the East End.
"We're concerned that Tower Hamlets will be little more than a glorified thoroughfare between the City of London and the Olympic Park."
A London 2012 spokesman said: "We proposed moving the start and finish of the London 2012 marathons for operational reasons and discussions were held and advice was sought from a range of organisations.
"It was a very tough decision for all the parties involved to take, but we believe it was the right one, and we believe the process undertaken to reach this decision was comprehensive, fair and legal.
"We understand that there has been disappointment in Tower Hamlets, and will continue to work with them to develop other opportunities for the borough to be part of the Games."