Birmingham City Council yesterday defended spending #51 million on agency staff - the highest out of any local authority in the country.
The GMB union described it as a "horrific abuse" of public money and urged all authorities to curb such spending.
Birmingham was followed in the league of big spenders by Ealing in London (#44 million) and Southwark in London (#40 million).
But a city council spokesman pointed out that the authority was three times as big as Ealing and four times larger than Southwark.
Most of the money went on social workers and supply teachers.
The GMB report shows 353 councils in the UK spent more than #1.1 billion last year, including #451 million by the London boroughs.
The union's annual conference in Blackpool called on all local authorities to cut back on their spending on temporary and agency staff recruitment.
National officer Brian Strutton said: "GMB members working in public services are rightly appalled at the waste of public money as a result of management's failure to put in place adequate levels of permanent staff to cover services to the public.
"Council spending on temporary and agency staff generally represents very bad value for money for the public.
"In most cases the only winners are the employment agencies who take a considerable slice of this money for administration and profits."
He said temporary staff were paid less while the council paid more to the agency than it would cost to employ workers directly.
The Birmingham City Council spokesman said: "We are the largest local authority in Europe, employing over 55,000 staff and serving one million residents, so it is inevitable that our overall expenditure on agency staff will be higher than other authorities.
"However, this expenditure is proportional and in line with the needs of any large city, with most of the money going on social workers, supply teachers and catering staff for events, rather than office workers.
"While we make every effort to minimise our use of temporary staff, those who we do employ play an invaluable role in helping us to deliver key frontline services such as education and social care. In order to ensure value for money and good standards, the council monitors and tracks agency staff expenditure closely."