The British government has been under pressure from European leaders to formally begin the two-year process of leaving the EU since it voted in June to leave the bloc.
Britain's shocking vote to leave the European Union just over a hundred days ago propelled Theresa May to power.
Since then, the new British prime minister has been under pressure to offer more details on her plan for departure from the 28-member bloc.
Now, in a change in tone from the former Remain campaigner, she says it is likely to happen in the next six months.
"It is also right that we should not let things drag on too long. Having voted to leave, I know that the public will soon expect to see on the horizon the point at which Britain does formally leave the European Union. So let me be absolutely clear: There will be no unnecessary delays in invoking Article 50. We will invoke it when we are ready, and we will be ready soon. We will invoke Article 50 no later than the end of March next year."
Using Article 50 of the EU's Lisbon Treaty will give Britain two years to then clinch one of the most complex deals in Europe since World War Two.
The president of the European Council, Donald Tusk, has welcomed the timing of the exit.
Mr Tusk says the other 27 EU states would engage to safeguard their interests once Article 50 is formally triggered.
Ms May made the announcement at the Conservative Party's annual conference in Birmingham.
She has told the meeting she will move next year to repeal the 1972 European Communities Act, a law that took Britain into what is now the EU.
"We will soon put before parliament a Great Repeal Bill, which will remove from the statute book once and for all the European Communities Act. This historic bill, which will be included in the next Queen's speech, will mean that the 1972 act, the legislation that gives direct effect to all EU law in Britain, will no longer apply from the date upon which we formally leave the European Union, and its effect will be clear. Our laws will be made not in Brussels but in Westminster. The judges interpreting those laws will sit not in Luxembourg but in courts in this country. The authority of EU law in Britain will end."
The Secretary of State for Exiting the European Union, David Davis, says he wants free trade with the EU on Britain's terms.
"We're looking at all the options, and we'll be prepared for any outcome. But it certainly won't be to anyone's benefit to see an increase in barriers to trade in either direction. So we want to maintain the freest possible trade between us without betraying the instruction we have received from the British people to take back control of our own affairs."
One of the biggest challenges in negotiating Britain's exit will be on immigration.
The free movement of labour is a founding principle of the EU, and millions of EU citizens live and work in Britain.
Foreign Secretary Boris Johnson says Britain is still committed to Europe.
"We may be leaving the EU, we are not leaving Europe. We will remain committed to all kinds of European cooperation at an intergovernmental level, whether it is maintaining sanctions against Russia for what's going on in Ukraine or sending our navy to help the Italians with the migrant crisis in the central Mediterranean, but we will also be able to speak up in our own distinctive voice."
But the leader of the Liberal Democrats party, Tim Farron, has condemned Ms May's decision.
He says she has acted too soon.
"What we're having today is a sign that Theresa May is now a prisoner of the right-wing, nationalist, protectionist, anti-business wing of her party, and, instead of looking after the business community and jobs and investment across Britain, she's chosen to do something against Britain's national interests in order to appease the right-wingers who run her party. That is not patriotic thing to do. It's a very foolish thing to do."
While the two-year timetable is mandated by the EU treaty, it can be extended by a unanimous vote of the remaining members of the bloc.
Once Article 50 is triggered, Britain will not be able to participate in internal EU discussions or decisions about its own withdrawal.
However, it will carry on taking part in other EU business as normal.
