Donald Trump has promised to win the White House in an electoral upset which would be “Brexit times 10” as the polls show Hillary Clinton’s lead narrowing.

Following fresh controversy over the former Secretary of State’s private email server, Ms Clinton’s lead slumped from 4.6 points to 2.5 points on Monday, according to the Real Clear Politics tracker which takes an average of most major polls.

The FBI announced on Friday that it had found thousands of emails connected to Ms Clinton’s work during an unrelated investigation into Anthony Weiner – a former Democratic congressman and estranged husband of top Clinton aide Huma Abedin – but they are not believed to be from the candidate herself.

Mr Trump is convinced he can still win despite polls in states like Pennsylvania, New Hampshire and Maine putting him between six and 10 points behind Ms Clinton.

He has taken inspiration from the shock vote to leave the EU following the referendum in June even though polls suggested the Remain campaign had maintained a narrow lead until polling day.

After weeks on the back foot following revelations that he had bragged about sexually assaulting women to an Access Hollywood reporter in 2005, Mr Trump has shifted his focus to several Democrat states in the hopes of winning last minute electoral college votes.

Speaking in Michigan, which last voted for a Republican in 1988, Mr Trump said: "I think we hit the motherlode, as they say. We can be sure that what's in those emails is absolutely devastating.

US election 2016: the race for the White House in pictures Show all 12 1 /12 US election 2016: the race for the White House in pictures US election 2016: the race for the White House in pictures Republican U.S. presidential nominee Donald Trump shakes hands with Democratic U.S. presidential nominee Hillary Clinton at the conclusion of their first presidential debate at Hofstra University in Hempstead, New York Reuters US election 2016: the race for the White House in pictures President Barack Obama embraces Democratic nominee Hillary Clinton on stage at the party's convention in Philadelphia US election 2016: the race for the White House in pictures Donald Trump's wife Melania delivered a speech at the GOP convention in Cleveland that was later found to have been cribbed in part from Michelle Obama's 2008 convention address AP US election 2016: the race for the White House in pictures Hillary Clinton talks to reporters aboard her new campaign plane on Labour Day, 5 September, her first 'press conference' since 2015 (Getty Images) US election 2016: the race for the White House in pictures Donald Trump held a joint press conference with Mexican leader Enrique Pena Nieto in Mexico City in August, hours before reiterating his harsh immigration plans at a campaign rally in Arizona Reuters US election 2016: the race for the White House in pictures Bernie Sanders officially endorsed Hillary Clinton, saying his progressive vision for ‘a transformed America’ would be ’best served by the defeat of Donald Trump’ Reuters US election 2016: the race for the White House in pictures Khizr and Gazala Khan appeared at the DNC to slam Trump for his stance on Muslim immigration, citing the case of their son Humayun Khan, who was killed in combat while serving as a Captain in the US Army in Iraq US election 2016: the race for the White House in pictures Libertarian candidate Gary Johnson is doing better in polls than any third party candidate since Ross Perot, 20 years ago Nicholas Kamm/AFP/Getty US election 2016: the race for the White House in pictures Green Party candidate Jill Stein (centre) marches with supporters in Colorado AP US election 2016: the race for the White House in pictures Hillary Clinton and Virginia Senator Tim Kaine at a rally in Kaine's home state in July, days before Ms Clinton tapped him to be her running mate Getty US election 2016: the race for the White House in pictures Trump on the campaign trail with his vice presidential pick, Indiana governor Mike Pence AP US election 2016: the race for the White House in pictures Former Ukip leader Nigel Farage appears at a Trump rally in Mississippi in August, where he told the crowd that he 'wouldn't vote for Hillary Clinton if you paid me'.

"Hillary is not the victim. The American people are the victims of a corrupt system. This is your one chance to change it. The Clinton crime spree ends on Nov 8."

With just a week to go he plans to spend millions of dollars in key Democrat states he thinks he can swing in his favour.

He will play on the recent rise in health care premiums associated with Obamacare which came into force this week and renewed mistrust of Ms Clinton following the FBI announcement, the Daily Telegraph reported.

Under the electoral college system – where states are assigned a number of delegates according to their population size – Mr Trump has to win several swing states and preferably a major Democrat state in order to win the race.