Sheffield Council is to spend £330,000 running virtually empty buildings left over from a regeneration scheme.
The money is being used to keep open Sorby House and Vestry Hall, which underwent multi-million pound renovations as part of the Burngreave New Deal for Communities project – but have never attracted the number of tenants hoped for.
Since 2011, the council has spent £1.2 million on insurance, running costs and staffing for the two buildings.
Now a further £333,664 will be spent in the 2013/14 financial year.
Opposition Liberal Democrats say the money could be used to keep up to eight of 15 threatened city libraries open instead.
Coun Andrew Sangar, Lib Dem finance spokesman, said: “Burngreave New Deal was a Labour vanity project, which has the left the city with a number of costly white elephants.
“Already £1.2 million has been blown on these buildings, which could have been used to protect services.
“It’s a simple question of priorities for the council – half-empty office blocks or treasured local libraries?”
The council says it is trying to make Sorby House viable by moving Burngreave Library into the Spital Hill building – if enough volunteers are found to keep the branch running.
Coun Harry Harpham, council deputy leader, said: “We are trying to ensure the buildings are sensibly used as part of our accommodation strategy which aims to save £30 million over 10 years by consolidating council departments in fewer buildings.
“The Lib Dems do nothing but carp and criticise.”