Law 1955 of 18 July 1991 established Attiko Metro SA as a private legal entity in the form of a public limited company. The government is the sole shareholder of AM but up to 49% of the shares are transferable and may be introduced in the Athens Stock Exchange. AM may merge with other mass transit organizations operating within the Attica Prefecture area, provided that at least 51% of the shares of the merged company remains with the Greek State.

As provided in the law, the purpose of AM is to oversee the design, construction, organization, administration, operation, exploitation and development of the subway network in the Attika Prefecture and cooperation with other mass transit agencies in the Attika Prefecture area. In the fulfillment of this purpose, all rights of the State arising from the Sepolia-Attiki Metro Tunnel Project and the contract for design, construction and commissioning of Lines 2 and 3 are transferred to AM. Law 1955 ratified and enacted into law the Construction Agreement for Lines 2 and 3.

On 29 September 1994 an amendment to the contract was ratified by the Greek Parliament with Law 2274/ 94.The Amendment modified, supplemented and clarified the contract provisions for better implementation of the contract terms to better serve the interests of both AM and OMC.

Until completion of the Project, AM supervised by the Minister of the Environment, Regional Planning and Public Works. The Minister provides all permits including building permits necessary to perform works within the scope of AM. At completion of the Project, the Minister of Transport and Communications has responsibility for supervision. The Minister of Commerce supervises the application of general provisions for the operation of public limited companies.

Law 1955 imposes obligations on the Archaeological Service and public utilities for their interface with the Project. Within 15 days after notification of the discovery of objects of archaeological interest, the Archaeological Service is to take the necessary measures to protect the antiquities and to recommend ways to continue AM works. The law also provides that real estate necessary for AM works is to be expropriated by joint decision of the Ministers of Finance AND Environment, Physical Planning and Public Works. Provided that normal use is not affected, owners of land overlying tunnels are not compensated. The Minister of Environment, Regional Planning and Public Works will specify the land required for temporary use.