DLA Piper is working with the the FBI and the UK National Crime Agency (NCA) to get its systems back online, as the firm enters its second day of outage following the ransomware attack that prompted it to shut down its computer and phone systems around the world.

The firm's email system is still out of action, while landline telephones are also still down as the firm moves to backup switchboards.

In a statement, the firm said it is working with external forensic experts and law enforcement authorities, including the FBI and the NCA, to recover from the attack and establish its source.

The statement said: "On 27 June, our advanced-warning system detected suspicious activity on our network, which, based on our investigation to date, appears to be related to the global cyber event known as 'Petya'. Our IT team acted quickly to prevent the spread of the suspected malware and to protect our systems."

It also said that the firm was aiming to get its email system up and running by this evening European time.

One partner at the firm said: "We are coping with it – we are collaborating with global authorities such as the FBI and trying to restore business as soon as possible.

"We have some systems that are operational, but we want to make sure when we connect properly everything is safe and we are trying to do that shortly.

"Emails are still not working, but we have reactivated the switchboards in many offices and we have backup switchboards in many countries redirecting calls."

Yesterday, partners told Legal Week that they had been instructed to turn off their computers as a precaution.

One London partner added: "People are managing surprisingly well and using personal emails. It's annoying. We're running around trying to keep clients happy. Clients are – so far – being understanding."

Multiple organisations have been affected by the hack, including Russian oil producer Rosneft and Danish shipping company Maersk.

A further statement, published on DLA's website adds: "We are currently dealing with a serious global cyber incident. We have taken down our systems as a precautionary measure which will mean you are currently unable to contact us by email or landline. Our people continue to be available on their usual mobile/cell phone numbers."