Freshfields Bruckhaus Deringer has secured a lead role advising BP on its attempts to ward off a takeover bid in a mandate that will be viewed as a coup for the City giant.

Freshfields was called in to advise BP on its takeover defence in the wake of the company's disastrous Gulf of Mexico oil spill, which began in April.

Speculation has mounted regarding BP's future after sustained political pressure in the US and a mounting clean-up bill, prompting a massive fall in the oil giant's share price. Exxon Mobil and Shell have been cited as potential bidders for the company.

BP confirmed this week that the clean-up had already cost the firm $3.12bn (£2bn). The company, which is reported to be currently seeking additional investment from the Middle East, has set itself against merging with an industry rival despite mounting debate over its future.

The Freshfields team advising BP includes veteran corporate partner Mark Rawlinson. Herbert Smith has also been retained in a secondary role.

The appointment, reported earlier today in The Times, will be seen as a surprise as Linklaters has traditionally been regarded as the bluechip oil company's lead corporate adviser.

BP has also instructed Kirkland & Ellis and Arnold & Porter to defend it against litigation in the US, which is expected to run into billions of dollars.

Freshfields, Herbert Smith and Linklaters declined to comment.