Article 50 is effectively the rule-book on how to leave the EU.
Once triggered, we would have two years to negotiate our exit from the union, as ITV News' presenter Tom Bradby explains.
Foreign Secretary Philip Hammond said there will have to be a "trade-off" on UK access to the EU single market and migration controls.
France's foreign minister called for the UK to trigger the exit clause after a meeting with counterparts of the six EU founding states.
European leaders have reacted to the UK's historic vote to leave the EU.