Tottenham Hotspur have demanded answers from Olympics chiefs ahead of possible court action over West Ham's successful bid for the 2012 stadium.
The Olympic Park Legacy Company (OPLC) confirmed it had received written questions from lawyers for the north London club.
The OPLC has four weeks to respond, after which Tottenham could request a judicial review.
A spokesman for London's mayor defended the stadium bidding process.
Tottenham may seek to challenge the process the OPLC followed in making its decision to choose West Ham as its preferred bidder for the stadium.
An OPLC spokeswoman said: "We can confirm that a letter before action in relation to potential judicial review proceedings has been issued.
"The Olympic Park Legacy Company ran a very rigorous and transparent process in its selection of the recommended preferred bidder.
"We have been supported by independent experts in their field in terms of legal, financial, commercial and technical advice."The spokeswoman added: "We have been consistent, fair, objective and entirely equal in our dealings with the bidders from start to finish.
"We are confident that if these judicial review proceedings are pursued, our approach will be entirely vindicated by the courts."
The UK government and the office of the mayor of London are joint owners of the OPLC and will both be involved in the court action.
The office of London Mayor Boris Johnson confirmed he had received a letter confirming Tottenham's decision.
A City Hall spokesman said: "We are advised and believe that the decision to choose West Ham as the preferred bidder was properly taken."'Number of concerns'
A statement released by Tottenham Hotspur said: "At this stage the club has not issued any proceedings against the OPLC or any other party in respect of the decision regarding the award of the Olympic Stadium.
"The club's lawyers have written letters to the OPLC, the Mayor of London, the Minister of Sport and the Secretary of State for Local Government and a separate letter to the London Borough of Newham raising a number of concerns with the processes which led to the award.
"The letters also requested - in the interests of transparency - for the provision of certain information concerning the processes, which the club considers that it is entitled to.
"Tottenham Hotspur will determine its next step as and when it receives a response to these letters."
Judicial reviews allow bodies to challenge decisions on grounds of illegality, irrationality and unfairness.