Apple (AAPL. O) is stepping up its fight with the British government to build a "backdoor" into iCloud, its most secure cloud storage system, by filing a legal complaint seeking to overturn the order, according to the Financial Times. Apple has appealed to the Investigatory Powers Tribunal, an independent judicial body that reviews complaints against British security services, according to people familiar with the matter. The court will consider whether the notice given to Apple by the UK is lawful and, if not, may order the notice to be withdrawn. The case is due to be heard as soon as this month. The government may limit the case on national security grounds. The UK government believes that breaking through encryption barriers in systems such as information and backups is essential to protecting the public from terrorist threats and investigating child sexual abuse.