Capita said it expected profits of at least £515m this year, down from its prediction in September of £535m-£555m which itself was a downgrade.
The company also said it would sell the majority of its Capita Asset Services division, as well as other assets.
Despite Capita's fall, the FTSE 100 index closed up 30 points at 6,932.
In the FTSE 250, shares in bookmakers were hit after MPs recommended that the maximum amount gamblers can stake on fixed-odds betting machines should be cut from £100 to £2.
Shares in William Hill dropped 7% and Ladbrokes Coral shares fell 4.5%.
Sports Direct shares were down 7.75% after the retailer reported a sharp fall in half-year profits.
Reported pre-tax profits fell 25% to £140.2m, with the company partly blaming the weak pound for the decline.
On the currency markets, the pound fell 0.49% against the dollar to $1.2564, and was 1.25% lower against the euro at €1.0618.
Shares in outsourcing group Capita have fallen more than 14% after the company issued another profit warning and said it would sell assets.