White & Case is in line for a key advisory role as oil giant Saudi Aramco plans a flotation that could value the company at up to $2.5trn (£1.7trn). 

Legal Week understands that the US firm is already advising the company informally as it assesses IPO options that could see it sell 5% of its equity for between $100bn and $150bn (£69bn-£104bn).

The much anticipated float, which is set to take place by 2018, has whet the appetite of banks as well as law firms, though the structure of the IPO, location of the listing and parts of the business to be included are all unclear. Last month, The Telegraph reported that Saudi authorities were planning a three-way listing in London, Hong Kong and New York.

So far, Saudi Aramco is understood to have enlisted the help of longstanding adviser White & Case as it finalises details of the listing. The firm would then be expected to have a central role when the process formally starts, should it go ahead, according to sources familiar with the deal.

"Aramco has being using White & Case to study the IPO but Aramco has yet to say it is doing an IPO so it isn't a formal appointment," said one source close to the oil major.

He added: "It will throw up quite a bit of legal work, such as compliance and restructuring."

It is understood that the underwriters have yet to be appointed, with that mandate therefore still up for grabs. 

Clifford Chance, Allen & Overy and Latham & Watkins are all expected to be contenders for this role.

If the sale were to take place as planned, it would reportedly be five times larger than any IPO in history.

Plans for the IPO were first aired by the Kingdom's deputy crown prince Mohammed bin Salman in January. The flotation forms part of his 'Vision 2030′ plan, which includes ambitious targets to generate growth and aid sectors that can help the oil-dependent economy to diversify.

White & Case has advised Aramco for decades. It has had an on-the-ground presence in Saudi since 1989 and has been advising clients there since the 1950s.

It remains the go-to adviser for the oil major, despite a raid by Latham & Watkins on its Middle East partnership six years ago, which saw it lose five partners.

In 2010, Latham hired a raft of partners from the firm globally, including a Middle East team that comprised corporate partner Villiers Terblanche, energy and infrastructure partner Nick Collins, projects partner Christopher Langdon, Riyadh office head Mohammed Al-Sheikh and Doha M&A specialist Andrew Macklin.

White & Case has experienced some upheaval in its Riyadh branch of late. In February, it announced that it had agreed a new tie-up in Saudi Arabia after its five-year association with Waleed Al-Nuwaiser came to an end in December 2015.

The US firm is now working with AlSalloum and AlToaimi. Zeyad Al-Salloum and Yazeed Al-Toaimi became association partners at White & Case. They were previously associates at the firm in Riyadh, with both joining the Riyadh office in 2013.

White & Case's 12-strong team of Riyadh lawyers includes three partners: Middle East head Doug Peel, energy partner Adam Pierson and capital markets partner Robert Vydra.

White & Case declined to comment. Saudi Aramco did not respond to a request for comment.