The Prime Minister said: "It is with strong conviction that I believe it is necessary to secure a strong and stable leadership this country needs.
"Division in Westminster will risk our ability to make a success of Brexit."
She added: "I have concluded the only way to guarantee certainty and security for years ahead is to hold this election.
"The country is coming together, but Westminster is not."
In response Tim Farron, the Liberal Democrat leader, said: "This election is your chance to change the direction of our country.
"If you want to avoid a disastrous hard Brexit. If you want to keep Britain in the single market. If you want a Britain that is open, tolerant and united, this is your chance.
"Only the Liberal Democrats can prevent a Conservative majority."
