Pinsent Masons has beaten magic circle opposition to win the key role on BAE Systems' $1.9bn (£1.01bn) IT outsourcing deal with CSC.
BAE invited Pinsents to join a competitive tender for the work, along with firms from the magic circle, and eventually chose the Pinsents team.
Partners David Isaacs and Bridget Edwards led the eight-strong Pinsents group advising BAE.
Pinsents managed to negotiate a contract with CSC that allows BAE to go back to the market to find new suppliers for elements of the programme if it is unhappy with CSC's performance.
"The big issue was whether or not to go to the market or stick with CSC," said Isaacs. "We established a contract with CSC which gave BAE a huge amount of flexibility. It's quite unique for a contract of this size."
CSC has managed BAE's IT since 1994 and the new agreement marks the third wave of the company's outsourcing programme. The company used its in-house team, led by Mike Phillips and Michael Woodfine.
With this deal completed, the IT team at Pinsents is now free to concentrate on the Home Office's e-borders programme. The original list has narrowed to a shortlist of four suppliers and lawyers expect to finalise a contract by September.