Dewey Ballantine has secured a leading role advising Greece's biggest wireless operator Cosmote on the acquisition of 42% of southeast Europe's leading mobile phone retailer Germanos for about $1.58bn (£836m).

The deal, understood to be a precursor to the launch of a full public offering for the remaining shares, sees the US firm advising the company on international finance aspects led by London capital markets partner Camille Abousleiman, assisted by associate Christopher Lenton.

Cosmote is a longstanding client of the firm, with Dewey having advised in 2000 on its IPO of about $130m (£69m), the largest Greek offering of that year.

On this deal, Cosmote was also advised on Greek law by local firm M&P Bernitsas with a team led by head of capital markets Nikos Papachristopoulos, together with head of European Union and competition Augustine Almyroudi.

Allen & Overy is under-stood to be advising Cosmote's financial adviser, JP Morgan.

Germanos is being advised on the deal by Greek firm Vassilios N Koutroubas & Associates.

Abousleiman told Legal Week: "We are advising [Cosmote] on international aspects of the deal. The acquisition could be followed by a tender offer for 100% of the company.

It is a big transaction by Greek standards and it is good to continue to assist a big client of ours."

The public offer is under-stood to be scheduled to take place after the end of August.

The deal will boost Cosmote's presence in Central & Eastern Europe via Germanos' operations in Bulgaria, Romania and Macedonia following acquisitions it has made of a network of stores in the region in recent years.