The consolidation of its international legal coverage is expected to begin early next year.
The move comes after The Post Office reduced its UK regional panel from 40 firms to four and its City firms from 12 to two practices in April this year.
The state-owned network also outsources work to Linklaters & Alliance, which advises its subsidiary German Parcel, bought by The Post Office in January for £256m.
The Post Office uses Clifford Chance on an ad hoc basis. But Ray Franks, head of finance and systems at The Post Office, says it has started using the firm less than it used to.
He says: "Clifford Chance lost out on commercial work on the last panel review in the UK earlier this year."
Franks adds: "The theory is that it will continue to do work that is ongoing. But when new work comes up the first port of call will be to [Slaughters, Camerons and Linklaters]."
Clare Wardle, assistant director in the intellectual property and technology department at The Post Office, says: "We have got an enormous range of firms [internationally]. On the IP side we use firms in every country in Europe and in countries abroad and in the US."
She adds: "Most of them we have acquired for historical reasons and that is one of the reasons we are going to review it."
Wardle says: "In the US we use two or three firms and again we are looking at if that is the most efficient way of doing it.
"For example we use a trademark firm there but we don't really use them for anything else so we should try to consolidate that.