Spending on agents' fees increased by £3.9million across the whole of the Football League last year.
Cardiff's fees were relatively low at £313,000, while Middlesbrough were the biggest spenders, splashing out £1,464,200.
FL Chairman Greg Clarke said: "Given the current economic climate, it is worrying to see such a significant amount leaking from the game.
"This year's figures demonstrate a considerable increase on those of last season and it is essential that clubs work to reduce this liability over the coming campaign."