Oil giant Exxon Mobil must hand over emails from an account once used by its former chief executive, Rex Tillerson, a judge has ruled.

Tillerson, now US Secretary of State, used an account under the name Wayne Tracker to discuss matters with other executives "expediently", Exxon's lawyers have confirmed.

However, in an ongoing investigation into whether Exxon misled its investors about its knowledge of climate change, Attorney General Eric Schneiderman alleges that Tillerson used the account to covertly discuss climate change and other sensitive topics at the company.

It is alleged that Exxon discovered the impact of climate change in the 70s, but worked to suppress the evidence and obfuscate the science.

After a 16-month court battle, Justice Barry Ostrager, of the State Supreme Court in Manhattan, on Wednesday (22 March) ordered Exxon to hand over the emails used by Tillerson, in addition to any documents previously subpoenaed by Schnierderman but not yet received.

An Exxon spokesman told CNN that the company had already handed over 400,000 documents, containing more than 2.6 million pages.

However, Schneiderman remained adamant that there were more, and, after his successful court hearing, Exxon has until 31 March to give up the remainder of the documents.

Schneiderman is particularly interested in emails from the Wayne Tracker account, but at the hearing lawyers for the attorney general said they were told by the company that many emails deleted themselves.

Tillerson left Exxon in December 2016 to take a place in US President Donald Trump's cabinet.