Herbert Smith Freehills (HSF) and Ashurst have picked up the lead legal roles on O2 parent company Telefonica's plans for an initial public offering (IPO) of the UK telecoms company.

If the float goes ahead, it looks set to be one of the largest London IPOs since Glencore's $10bn offering in 2011. The size of the stake Telefonica plans to list is not yet clear, although a figure of 30% has been reported.

Longstanding Telefonica adviser HSF has picked up the coveted role advising the company on the flotation, with equity capital markets head Charles Howarth understood to be at the helm. Howarth joined HSF from Linklaters in 2002 and has since advised on listings including Kennedy Wilson Europe Real Estate's £1bn IPO in 2014.

HSF also advised Telefonica on CK Hutchison's recent £10.5bn takeover bid for O2, which was blocked by European Commission competition authorities earlier this year. The bid, which would have seen O2 merge with Hutchison's Three UK business, saw Linklaters, Freshfields Bruckhaus Deringer and Baker & McKenzie advise Hutchison.

Ashurst is understood to be advising the underwriting banks on the proposed float, with equity capital markets head Nicholas Holmes leading the firm's team. Holmes was part of the Ashurst team that advised Merlin Entertainments Group on its £4bn IPO in 2014, while last year he advised UK IT infrastructure provider Softcat on its £470m IPO.

HSF and Ashurst declined to comment.

Yesterday (4 October) Legal Week reported that Freshfields Bruckhaus Deringer and Linklaters have won roles on the flotation of medical device maker ConvaTec, the biggest confirmed IPO since the June vote to leave the European Union.

ConvaTec, which makes medical products including colostomy bags, announced earlier this week that it intends to float on the London Stock Exchange by early November. The flotation is expected to raise approximately $1.8bn (£1.4bn). Freshfields is acting as lead adviser to the company, while Linklaters is acting for the banks.