Philip Hammond, the Foreign Secretary, has warned that it could take four years for Britain's exit from the European Union to be agreed.

Negotiations for Britain's exit can only start once Article 50 of the Lisbon Treaty is triggered. This process will take two years but agreeing future treaties could take an additional four years.

Asked by Alex Salmond, the SNP leader, whether it would take longer to negotiate Brexit than World War II, Mr Hammond said: "If a future treaty between the UK and the EU 27 is deemed to be a mixed competence it will have to be ratified by 27 national Parliaments.