Theresa May attempted to change the focus of the general election campaign back to the UK's split from the EU as she visited the north-east of England on Thursday 1 June.
The prime minister's speech came just a week before the general election on 8 June. May called for the vote in April in a bid to boost her negotiating hand during the two-year-long Brexit talks with the EU.
But the Conservatives have seen their double-digit poll lead over Labour plummet over the course of the campaign.
A YouGov survey, of more than 1,800 people between 30 and 31, gave the Tories a three point lead (42% versus 39%). The figures suggest May's majority could be reduced and the UK would face a hung parliament.
"You can't negotiate the right Brexit deal for Britain if you don't believe in Britain," she said.
"You can't fight for Britain if you don't have confidence in our strengths and in all that we have to offer. You can only deliver Brexit if you believe in Brexit. You can only fight for Britain if you believe in Britain.
"You can only deliver for Britain if you have the strength, the plan and the determination to see it through."
May, who campaigned for Remain at the EU referendum, added: "And what we know in this election is that the only other person that can be prime minister in seven days' time is simply not up to the job. He doesn't believe in Britain. He doesn't have a plan. He doesn't have what it takes.
"And after last night it's clearer than ever that just 11 days after the election when the negotiations begin, Jeremy Corbyn's focus wouldn't be on trying to negotiate a deal for Britain in Europe, but on trying to stitch up a deal with Nicola Sturgeon and the rest."
Former Deputy Prime Minister Nick Clegg, the Liberal Democrat Europe spokesman, described May's speech as "delusional".
"The relentlessly upbeat assessment of Brexit in her speech today is a U-turn of epic proportions," he said. "It is also dangerous, because it is calculated to distract attention from the shark-infested waters which we are now entering, and the fact that Theresa May herself has steered us towards them.
"What comes next is not the sunny picture described in this delusional speech. Negotiating Brexit will be task of monumental proportions which - even if handled expertly - will do great damage to our prosperity, the state of our public services and Britain's place in the world.
"The Liberal Democrats will give you the chance to change Britain's future by allowing you to reject a bad Brexit deal and remain in the EU."
Labour leader Jeremy Corbyn is also expected to deliver a major speech on Brexit on Thursday. The left-winger is expected to warn against walking away from the divorce talks with no deal.
"No deal on Brexit is the worst of all deals," Corbyn will argue. The Labour chief has also promised to guarantee the residency rights of the more than three million EU nationals living in the UK.