iPhone (4)

Some of the world's biggest tech companies have given their backing to Apple in its legal battle with the FBI over unlocking an iPhone.

Twitter, AirBnB, Ebay, LinkedIn, Reddit, Amazon, Cisco, Facebook, Google, Microsoft, Mozilla, Pinterest, Snapchat, WhatsApp and Yahoo have all given formal backing to Apple.

Two separate groups have filed an amicus brief - which allows parties not directly involved in the case, but could be affected by the outcome, to have a say.

In February, Apple received a court order demanding it helps the FBI to unlock the iPhone of Syed Rizwan Farook, the gunman who killed 14 people in the San Bernardino terror attack in December.

As part of Apple's security measures, a user can choose to erase all files on an iPhone if an incorrect password is entered 10 times. It is understood Farook used a four-digit password, meaning there are 10,000 possible combinations.

The FBI wants Apple to allow it to enter an unlimited number of passwords without the files being deleted. And it wants Apple to implement a system that allows it to enter all passwords automatically, rather than entering 10,000 possibilities manually.

Shortly after receiving the court order, Apple appealed the decision, describing it as an "unprecedented step which threatens the security of our customers".

The FBI believes Farook's iPhone could have information vital surrounding the attack.