A development partnership between Allied London and Manchester City Council has been agreed to ensure the future development of the office complex Spinningfields in the city centre.
The city council has acquired the final plots at 1 and 2 Hardman Square and 2 and 3 Hardman Boulevard from developer Allied London.
The extension of the agreement will allow Allied London to develop another minimum of 580,000 sq ft of office and retail space.
Manchester City Council and Allied London have agreed to extend the development agreement to December 2015 with Allied London being granted a 250-year lease on the two plots at a commercial ground rent.
Under the terms of the new 250-year lease Allied London has to start development on the two plots by 2015 but they have separate start dates of 2011 and 2013.
The sum invested by Manchester City Council was not disclosed.
Sir Howard Bernstein, chief executive of Manchester City Council, said: "This is a prudent investment by the city council. It's important to build a platform to maintain growth and respond to the demands for office space in the city centre."
Michael Ingall, chief executive of Allied London, added: "This agreement has been reached first and foremost to consolidate an already successful partnership and extend the partnership to reflect what is going to be a longer development period than originally envisaged.
"The agreement also seeks to consolidate the city council's investment in Spinningfields and will enable us as developer to more freely fund future development.
"The city council's investment provides a commercial return but importantly also gives the city the option of being a long-term investor in this important new city business quarter long after we as the developer have completed it.
"My objective has always been to ensure that at some stage long-term ownership is put in place to thereby protect long-term value for the estate. This is a good time to do it and Manchester City Council is the best partner to do that with."