* Crozier to become chairman on Dec. 1

* BT in crucial transformation phase

LONDON, Aug 17 (Reuters) - BT named Adam Crozier as its next chairman on Tuesday, tapping one of the most experienced corporate operators in Britain to oversee the group through a key transformational period in its 175-year history.

Crozier, who previously brought new strategies to broadcaster ITV, Royal Mail and the Football Association, will become chairman designate on Nov. 1 and chairman proper a month later, when Jan du Plessis retires.

He joins as the company embarks on a major infrastructure programme, racing to take fast broadband fibre to 25 million premises by the end of 2026, and as it weighs partnering with investors on part of the build, and in its sports TV division.

He will also have to manage the new relationship with Patrick Drahi, a Franco-Israeli telecoms entrepreneur who stunned investors in June when he announced that he controlled 12.1% of BT, making him the biggest shareholder.

Crozier will work alongside BT CEO Philip Jansen, dubbed by observers as a "man in a hurry", who having received millions of pounds in his previous job running Worldpay, is battling to shake up and modernise the former telecoms monopoly.

BT was forced earlier this year to deny that Jansen had threatened to resign unless du Plessis quit, following a media report that the CEO was frustrated with the pace of change.

A person familiar with the matter said the two had discussed whether they were the best combination to deliver such widescale change but that their relationship had always remained amicable.

Iain Conn, BT's senior independent director, said Crozier had been the board's unanimous choice to take over.

"He has significant experience in leading public company boards, developing teams and managing stakeholders and brings a strong transformational and operational track record in large-scale executive roles," he said.

Crozier will step down as chairman of online fashion retail ASOS on Nov. 29, and as a non-executive director of Sony Corp. at the end of the year.

"BT is a hugely important company, with a critical role to play in building the digital networks and services to support the UK's future," Crozier said.

BT's shares, up 64% in a year on favourable regulatory rulings and the Drahi stake, slipped 1% in early trade. (Reporting by Kate Holton Editing by Michael Holden and David Holmes)