MegaFon , Russia's leading mobile phone operator, has submitted a request to the national markets regulator for permission to list proxy shares in London, bringing it a step closer to a major initial public offering.

MegaFon, in which Russia's richest man Alisher Usmanov took control in a complex deal in April, had been eyeing the float of a 20 percent stake which could be worth as much as $4 billion (2.5 billion pounds), sources familiar with the matter said at the time.

The company is requesting to list 123 million shares, it said in a filing on Tuesday. That works out at 19.9 percent of its 620 million shares outstanding.

Megafon is majority owned by Usmanov's AF Telecom which holds 50 percent plus one share of the company and 35.6 percent is owned by Nordic telecoms firm Teliasonera , while 14.4 percent is held as treasury stock.

At the time of the April deal, the parties announced that Teliasonera would reduce its holding in MegaFon to 25.1 percent through the proposed IPO, while the remainder of the offering would be made up of treasury shares.

Megafon was not available for comment.

(Reporting by Douglas Busvine; Editing by Megan Davies)