The European Commission, whose chief negotiator Michel Barnier will spearhead the talks with London on behalf of the other 27 member states, said it was ready for the Brexit process.
May's government said her permanent envoy to the EU had informed European Council President Donald Tusk of the date when Britain intends to invoke Article 50 of the Lisbon Treaty - the mechanism for starting its departure from the bloc.
The Pound remained steady against the dollar and the euro as Downing Street confirmed the news.
A spokesperson for the European Commission said it was "ready to begin negotiations" and it had "everything ready and decided".
In response to the news, Mr Tusk tweeted: "Within 48 hours of the United Kingdom triggering Article 50, I will present the draft Brexit guidelines to the EU27 Member States".
A spokesman said the government wants negotiations to start as soon as possible but added that they "fully appreciate it is right that the other 27 European Union states have time to agree their position". He will then the send the Council his recommendations for how to run the negotiations.
Article 50, which has never been used before, is quite simple and only consists of five short paragraphs.
Some analysts are warning of the potential for traders to be forced out of those stretched short positions if the exchange rate moves against them, which could prompt volatility.
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That revelation shocked the United States , and has been credited with helping Donald Trump win the election and become president. The account says it is managed by Trump's social media manager, Dan Scavino. "We have no information that supports them".
So what do we look out for as article 50 is triggered and in the immediate aftermath? Next Wednesday, the Government will deliver on that decision and formally start the process by triggering Article 50.
The PM is due to address MPs in a statement to the House of Commons following her regular weekly session of Prime Minister's Questions on 29 March.
"We think the imminent activation of Article 50 will trigger hard trade negotiations, which is not properly priced into the currency", Michael Cahill, New York-based strategist at Goldman Sachs, wrote in a note to clients.
Officials from the U.K.'s Department for International Trade are investigating the possibility of keeping tariffs between Britain and the European Union at zero as part of an interim arrangement that could last up to 10 years, allowing more time for a full trade deal to be negotiated after Britain has left the bloc, according to sources familiar with the discussions that have taken place at the World Trade Organization.Under a little-known WTO clause, the United Kingdom and Brussels would be allowed a "reasonable length of time" after Brexit to agree a free-trade deal before trade law would force both sides to impose the same tariffs on each other as they do on everybody else.
News of the Article 50 date - known as "B Day" among expectant Euroskeptics - was a closely guarded government secret.
"I don't think there are many countries that would have anything critical to say of such a deal as they would be helped by it", he added.
Tim Farron, leader of the Liberal Democratic Party, said, "Theresa May is embarking on an extreme and divisive Brexit".
Preparations are understood to be highly advanced, with the letter to Mrs Tusk already drafted and Britain's opening demands in the Brexit negotiation prepared.