The negotiations are expected to be tough and will likely take place over two years, though the more complex aspects of Britain's future relationship with the European Union, such as trade, could take even longer.
"The Liberal Democrats have said they want to grind the business of government to a standstill".
Speaking outside her Downing Street residence in London, May warned that "division in Westminster will risk our ability to make a success of Brexit".
May will ask the House of Commons to vote on the snap election on Wednesday.
The 60-year-old vicar's daughter is Britain's second prime minister after Margaret Thatcher and many commentators have drawn comparisons to the steely determination of the "Iron Lady".
She said: "Our opponents believe that because the Government's majority is so small, our resolve will weaken and that they can force us to change course".
One of the reasons her announcement of an early election in less than two months from now was such a surprise is that both Downing Street and Theresa May herself have repeatedly denied that there will be one.
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Before she was designated the new Prime-Minister of Great Britain, during her 30 June speech, Theresa May declared that during her term "snap elections" will never take place.
The decision is important because it suggests May will have little trouble securing enough votes to overturn the Fixed Parliaments Act, which set the date for the next election for 2020. She added that her decision wasn't "about political games" but "about what is right for the country". "There is not going to be a general election".
May's Conservatives, who were split on the issue of European Union membership ahead of last year's referendum, are now far ahead of Labour, the main opposition party, according to opinion polls.
The prime minister said that if an election was not held soon, "the negotiations with the European Union will reach their most hard stage in the run-up to the next scheduled election", she added, "Division in Westminster will risk our ability to make a success of Brexit and it will cause damaging uncertainty and instability to the country".
May's office says she will speak after the weekly meeting of her Cabinet. She said that despite predictions of immediate financial and economic doom, Britain had seen "consumer confidence remain high, record numbers of jobs and economic growth that has exceeded all expectations".
Under legislation introduced by the coalition government led her predecessor, David Cameron, an early election requires the support of two-thirds of MPs in the House of Commons.
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