Kemp Little founding partner Richard Kemp said: "Jon's made a huge contribution to the practice. We're confident that there won't be any client departures. We've spoken to them all. The Simmons business model is very different to ours. We don't compete with Simmons."
Little's clients include US services giant EDS, Simpay and Standard Chartered Bank.
Barrett said: "We've been planning to significantly expand our IT and outsourcing practice and this is the first step. Jonathan's very competent and has a very substantial practice.
"In terms of technology and outsourcing work, the institutions are pretty large and therefore they tend to have a particular pricing model that applies. I don't think that the Simmons and Kemp Little pricing models are particularly dissimilar."