Milbank has lost nine of its lawyers, including five partners, to the two firms in the past month.
Nicholas Buckworth, Kenneth MacRitchie and Stephen Peppiatt leave for Shearmans' London office and Calvert Miller moves to its Singapore office in December. The four are all project finance partners.
Diana Downing has also moved from Milbanks' London office, but has joined Allen & Overy, whose Moscow office has also poached two Russian lawyers from Milbanks' practice in Moscow.
The Allen & Overy moves are understood to have been prompted by the departure of Millbanks' former Moscow head David Slade to join Allen & Overy's New York office.
The moves have left Milbanks' London office with just one partner, its head, Philip Fletcher. He said the firm was looking to recruit some “top English lawyers”.
Fletcher added: “The rapid movement of lawyers between top firms is becoming increasingly par for the course.”
Allen & Overy's Moscow team now has 14 fee earners. Its head, Doran Doeh, said there was so much work coming in that firms were not even having to compete.
The four lawyers poached by Shearmans form part of the firm's plan to start practising English law throughout the world. They will join the practice as partners in the project finance department.
Stephen Volk, Shearmans' senior partner, said: “This is about globalisation, not just practising English law in the UK. The national borders to capital have dissolved.”