The council has been challenged to honour a pre-election pledge on funding the Upper Norwood Library.
At a cabinet meeting on Monday, Croydon Council agreed to offer assistance to the community to form a group to take over running a library service run by a joint committee of Croydon and Lambeth councillors for 112 years.
Croydon is offering an annual grant of £75,000, £112,000 less than it currently spends on the service and £95,000 less than Lambeth Council.
Shadow cabinet member for culture and sport Councillor Tim Godfrey emphasized this broke a Conservative election pledge made in 2010.
In the leaflet published before the local elections two years ago, the Conservatives state: "We are pledging, at a minimum, to match Labour Lambeth’s current contribution to the library. This funding will enable the excellent work done at the library to continue."
In 2010 Lambeth Council contributed more than £200,000 to the running of the library.
Coun Godfrey said: "Why does the council think it is acceptable for Lambeth Council to pay for Croydon service users. You felt it was important enough to put this pledge on the front page of your election document, but now two years on you offer a fraction of the cost."
Councillor Tim Pollard, cabinet member for children families and learning, said: "What that refers to is the Upper Norwood Libaray Join Agreement and was put forward before Lambeth breached the terms of the agreement and effectively tore up the agreement. It doesn’t exist and it is not appropriate to go back."
Campaigners hoping to keep the service open have set up a trust to take control of the library in April next year.