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British firms need greater clarity about the Brexit process, the Confederation of British Industry warned as Theresa May said she would fire the starting gun by the end of March next year.
The prime minister used a speech on the opening day of the Conservative Party conference in Birmingham to confirm Article 50 would be triggered by the end of March, starting the two-year countdown to Brexit.
CBI director general Carolyn Fairbairn warned businesses "cannot continue to operate in the dark" about Mrs May's intentions for the future relationship with the European Union (EU).
She said: "With a rapid timetable pointing to an exit from the EU in spring 2019, businesses need to know the Government's ambition on the fundamental issues of skills and barrier-free access to EU markets as soon as possible.
"With the Great Repeal Bill we now know that on the day the UK leaves the EU, the rules businesses must follow will be broadly the same as they are today.
"But businesses cannot continue to operate in the dark in other areas. The decisions they face today are real and pressing.
Theresa May has vowed "to get on with the job" of kick-starting divorce proceedings with the European Union by the end of March next year.
In her first Conservative party conference speech as prime minister, she also dismissed claims that Britain would give away control of immigration in return for a better trade deal with the EU.
But her timetable for leaving is causing a split within the party.
ITV News Deputy Political Editor Chris Ship reports:
Prime Minister Theresa May has said she will not accept any limits on Britain's ability to control immigration in negotiations to leave the European Union which will start by the end of March 2017.
Speaking at the Conservative party conference in Birmingham, Mrs May rejected the argument Britain must choose between "hard Brexit" - in which it regains control over immigration but loses access to the European single market - and "soft Brexit", under which access to the single market comes with a requirement to allow free movement of EU workers.
"I know some people ask about the 'trade-off' between controlling immigration and trading with Europe. But that is the wrong way of looking at things.
"We have voted to leave the European Union and become a fully independent, sovereign country.
"We will do what independent, sovereign countries do. We will decide for ourselves how we control immigration. And we will be free to pass our own laws."
Nicola Sturgeon has accused Theresa May of ignoring Scotland's "voice and interests" as Britain prepares to leave the European Union.
The prime minister told the Conservative party conference in Birmingham there is "no opt out from Brexit" and she will "never allow divisive nationalists to undermine the precious union between the four nations of our United Kingdom".
"We will negotiate as one United Kingdom and we will leave the European Union as one United Kingdom," said Mrs May.
But the First Minister of Scotland hit out at Mrs May's remarks, after the majority of people in Scotland voted to remain in the EU, unlike the rest of the UK.
Ms Sturgeon said on twitter the prime minister was "going out of her way" to say that "Scotland's voice and interest don't matter".
She added that Mrs May's "approach" was "strange" for "someone who wants to keep the UK together".
Theresa May has confirmed plans for a Great Repeal Bill to repeal the 1972 Act of Parliament which took Britain into what was then the EEC, and to transpose EU laws into domestic law.
She insisted she will aim to strike a deal with former EU partners to include "co-operation on law enforcement and counter-terrorism work ... free trade in goods and services" and "to give British companies the maximum freedom to trade with and operate in the single market and let European businesses do the same here".
"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 on which we formally leave the European Union," she said.
"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."
There will be no unnecessary delays in invoking article 50, Theresa May has said.
Speaking at the Conservative party conference, the prime minister said: "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 clear. There will be no unnecessary delays in invoking Article 50."
Confirming the date by which this will happen which she had already stated in an earlier interview, Mrs May added: "We will invoke it when we are ready and we will be ready soon. We will invoke Article 50 by no later than the end of March next year."
Theresa May has said "the sky has not fallen in as some predicted it would" following the vote to leave in the EU referendum.
Speaking at the Conservative party conference in Birmingham, the prime minister also defended her decision not to invoke Article 50 before the end of the year.
Earlier on Sunday, Mrs May confirmed Article 50 of the Lisbon Treaty, which triggers the formal withdrawal process for Brexit, will be invoked by the end of March 2017.
"That decision means we have the time to develop our negotiating strategy and avoid setting the clock ticking until our objectives are clear and agreed," she said.
"And it has also meant that we have given some certainty to businesses and investors. There is still some uncertainty but the sky has not fallen in as some predicted it would. Our economy remains strong."
Conservative MP Anna Soubry has voiced concerns after Theresa May's announcement that the formal process for Brexit will be triggered before the end of March 2017.
The former business minister, who has campaigned for the UK to remain in the EU, told Peston on Sunday: "Triggering Brexit as early as March really concerns me...troubles me hugely.
She added: "I'm sorry, it's going to take a lot of time and effort to disentangle ourselves and get the right deal."
"This idea that we hold the cards and that the EU is going to come to us and say 'you know what, we'll give you pretty much what you want'...is rubbish."
"We're going to get something worse...and we don't hold the cards the EU does," she added.
The government will seek to keep some EU laws in the areas of environment and workers' rights post-Brexit, Transport Secretary Chris Grayling has said.
Speaking on ITV's Peston on Sunday, Mr Grayling said the public wants certainty on the issue of Britain leaving the EU, and endorsed Theresa May's announcement that Article 50 will be triggered before the end of March 2017.
"It's important that we give some certainty about the process," he said.
"Today's announcements are a staging post on the route towards triggering Article 50 and towards holding the negotiations."
The Transport Secretary also said that Britain would have a "sensible" agreement with the EU that allows the UK to continue to trade with the continent; and take control of the flow of people into Britain.
Latest ITV News reports
The reaction within the Conservative Party to her latest announcements illustrates the problem Theresa May has.
Theresa May has confirmed Article 50, which triggers the withdrawal process for Britain leaving the EU, will be invoked by the end of March.