The UK government faces a legal challenge over a visa system highlighted by a Financial Times investigation that leaves migrant crew on British fishing boats vulnerable to trafficking and modern slavery.

Human rights lawyers have begun the process of bringing a judicial review against the Home Office, arguing that visa arrangements used to employ overseas crew are in breach of European human rights law...

While boats operating in UK waters must have skilled worker visas for migrant crew, vessels that catch fish in international waters are legally entitled to use the transit visa scheme — widely viewed as an immigration “loophole” — to employ migrant workers. The crew, who do not have permission to work in the UK, live onboard all year round to avoid violating the law when their boats return to port...