The UK telco claimed that the purchase will "significantly enhance" its consulting capabilities, and will see "hundreds of consultants" joining BT mostly in the US and Canada.
INS specialises in enterprise architecture and governance, business productivity, information risk management and infrastructure transformation.
Based in Santa Clara, California, and with offices in Europe, Asia and North America, INS builds, implements and secures business technology infrastructures.
Andy Green, chief executive at BT Global Services, said: "The INS business is another great services acquisition for BT. INS has a proven business model in the networked IT services space that we will replicate around the world."
The deal follows BT's acquisition of Counterpane in October 2006.
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