File Photo: UK PM Boris Johnson Photograph:( Reuters )
'We're fighting for every vote and... this is a critical moment for this country, the choice on Thursday is unbelievably stark,' Johnson told British media.
The prime minister is hoping to regain the majority that his Conservative Party lost in 2019, with a slew of polls released on Saturday suggesting he was narrowly on course to achieve his goal.
The stakes could barely be higher, with the fate of Brexit still to be settled.
"We're fighting for every vote and... this is a critical moment for this country, the choice on Thursday is unbelievably stark," he told Sky News.
He compared the election to other "historic elections" such as 1906, 1945 and 1979.
"The impact of this election will be felt for decades to come," he wrote in an open letter.
A Conservative majority would almost certainly cement Britain's departure from the European Union, but any other result could lead to a second referendum and the 2016 result being overturned.
The prime minister faced scrutiny over promises there would be no checks on goods moving between Northern Ireland and mainland Britain as part of his Brexit deal with Brussels.
"There's no question of there being checks," he told Sky, adding "why would we put checks on goods going from NI to GB or GB to NI? It doesn't make sense."
His claims appear to contradict a leaked government document revealed by Labour leader Jeremy Corbyn this week, which suggested that businesses would need to fill in paperwork when moving goods across the Irish Sea.
Corbyn is banking on a late surge, as he achieved in 2017, to prevent Johnson from winning a majority.
'Bear down on migration'
But polls released on Saturday showed the Conservatives with an average lead of 10 per cent.
Pollsters believe that Johnson needs to be more than six points clear of Labour on Thursday to secure a majority.
The prime minister announced Sunday that he would introduce an "Australian-style" points based immigration system if elected, signalling a tightening of low-skilled immigration.
"I've said that what we want to do is bear down on a migration, particularly of unskilled workers who have no job to come to," he said.
"You've got to have a system by which party politicians can say to people, 'well, yes, we are letting people in but we're doing it in a way that is controlled'."
Exit polls from Thursday's vote will be released at 2000 GMT, but the final result will not be confirmed until Friday.