The Channel Tunnel is covered by the Sangatte Protocol (1991) and the Additional Protocol to the Sangatte Protocol (2000), which together enforce pre-boarding i

The Channel Tunnel is covered by the Sangatte Protocol (1991) and the Additional Protocol to the Sangatte Protocol (2000), which together enforce pre-boarding immigration controls. (For comparison, the Treaty of Le Touquet introduced them for cross-Channel ferries in 2003.) Ultimately British politics has driven all of this: it's much easier, legally, to refuse someone without documentation at the UK border control in France (or Belgium, or in the future Holland) than it is when they claim asylum upon arrival in the UK.