The Department of Environment, Food and Rural Affairs has signed a seven-year deal with IBM to provide IT and business change services.
The department said it had contracted IBM to "deliver an IT-enabled strategy for change, improving the department’s internal process leading to better services for customers".
The deal is worth more than £400m over the lifetime of the contract.
Around 330 Defra IT staff will transfer to IBM when the contract begins in October. The global IT company is already in discussion with unions over transfer arrangements.
In procurement process has been fraught with difficulty. The deal was expected to be signed by the middle of last years, but was delayed several times, in part because Defra was recruiting a new IT director.
The Public and Commercial Services union also threatened strike action during the procurement process over fears of job losses.