EU referendum, brexit and the aftermath

https://en.wikipedia.org/wiki/Fixed-term_Parliaments_Act_2011

Section 2 of the Act also provides for two ways in which a general election can be held before the end of this five-year period:

1. If the House of Commons resolves "That this House has no confidence in Her Majesty's Government", an early general election is held, unless the House of Commons subsequently resolves "That this House has confidence in Her Majesty's Government". This second resolution must be made within fourteen days of the first.
   
2. If the House of Commons, with the support of two-thirds of its total membership (including vacant seats), resolves "That there shall be an early parliamentary general election".
   
   

In either of these two cases, the Monarch (on the recommendation of the prime minister) appoints the date of the new election by proclamation. Parliament is then dissolved 25 working days before that date.