British newspaper The Morning Star reported that the tribunal would hear evidence later this year from a group of exiles who are calling on the Home Office to lift the ban on the Liberation Tigers of Tamil Eelam (LTTE).
The government department wants much of the case to be heard in secret at the seldom-used Proscribed Organisations Appeals Commission, The Star added.
In 2014 the European Court of Justice said an EU ban on the Tamils should be scrapped. Britain was one of the few EU member states that sent lawyers to Luxembourg in an unsuccessful attempt to maintain the continent-wide ban.
Tamil activists in London are now focusing their efforts on the ban in Britain, which remains in force.
The challenge has been lodged by the Transnational Government of Tamil Eelam (TGTE), which is represented by Professor Conor Gearty of Matrix Chambers.
The TGTE said that the ban has a chilling effect on Tamils demanding self-determination.
The deproscription process is notoriously tough, although the International Sikh Youth Federation was unbanned in 2016, meaning that membership of the Sikh separatist group is no longer an offence in Britain.