The AIM-listed cloud computing company said the deal would be worth £31,000 a month over three years.
In a statement it said the deal had come from a contract win with a FTSE 100 company by one of its “key channel partners”.
“It will mean that 48,500 individual end-users within three FTSE 100 companies will be reliant on Outsourcery’s services”, said the firm.
Linney said: “I am proud that we can now count three of Britain’s largest public companies as end-customers and look forward to working with our partners to add to that number as we prove our capabilities and extend our market leadership position.”
Last month the company saw its share price increase by more than 50% following an announcement that it has been named as a Microsoft Cloud Solution Provider.