Slaughter and May and Freshfields Bruckhaus Deringer have won lead roles advising on Shell's £47bn acquisition of natural gas producer BG Group.

The two firms have agreed on a cash and shares offer from Shell that will pay BG Group shareholders around 1,350p per share – a 50% premium on BG Group's share price on Tuesday (7 April).

The acquisition is due to complete in early 2016.

Anglo-Dutch oil major Shell said the takeover would boost its oil and gas reserves by 25% and its production capabilities by 20%.

Slaughters is advising Shell with a London team led by corporate and commercial partners Roland Turnill, Hywel Davies and Rebecca Cousin.

De Brauw Blackstone Westbroek is acting for the oil giant on Dutch legal matters, led by partner Paul Sleurink, while Cravath Swaine & Moore is advising on US corporate matters, led by partners William Rogers and Richard Hall.

The trio of Slaughters, De Brauw and Cravath won roles on Shell's historic restructuring in 2004, which has been described as one of the UK's biggest ever transactions.

All three firms worked with the FTSE 100 company providing UK, Dutch and US advice on the move that saw the combination of the Netherlands-based Royal Dutch and UK-based Shell to create Royal Dutch Shell.

On today's deal Freshfields is advising BG Group with a London team led by M&A partner Mark Rawlinson and corporate partner Graham Watson.

BG Group was created in 1997 when British Gas demerged into two separately listed companies, with Centrica having responsibility for the retail side of the business.

BG Group shareholders will end up owning roughly 19% of the combined group.