Mr Hague, now a member of the House of Lords was the Tory leader from 1997 to 2001. Lord Hague has said Mr Johnson will face the toughest challenge of any Prime Minister since Winston Churchill succeeded Neville Chamberlain amidst the Second World War. He wrote in his Daily Telegraph column: “You carry the hopes – even of those of us who didn’t vote for you – that you will defy the odds to keep both country and party intact and successful.
“But you face the most immediately daunting challenge on entering No 10 since Churchill, who you have studied so attentively, stepped over its threshold in 1940.”
He added later: “A British leader needs to be part of binding the West together, or Brexit will one day be seen as the prelude to a more catastrophic disintegration.”
The threat of the UK breaking up was also reiterated: “And never forget that you are leading a United Kingdom in danger of breaking up.”
Mr Hague, who was Foreign Minister for a time under David Cameron, did offer a somewhat vote of confidence to Mr Johnson: “One of the main reasons you will have been chosen is that you did a remarkable thing: you were twice elected as a Conservative Mayor of London.
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“The philosophy you espoused then, as a fiscal conservative and social liberal, is the route to maintaining the Tories as a broadly-based party of Government.”
Mr Johnson is facing Foreign Secretary Jeremy Hunt in a ballot of Tory Party members.
The former Foreign Secretary has long been the bookies’ favourite for the position and started the race as favourite and not lost the position at the top of the podium.
When Lord Hague succeeded Sir John Major as Tory leader, Ken Clarke started as favourite in the race.
He did not come top in the first or second ballot of Tory MPs, that went to Mr Clarke.
Mr Hague polled at second in both of the ballots, though Mr Johnson has consistently led the way.
Under his leadership at the 2001 general election, the Tories gained just one extra seat, resulting in Mr Hague’s resignation.
Iain Duncan Smith, Mr Johnson’s campaign chief, took over, despite Michael Portillo starting as favourite, but would not contest a single general election after losing a no-confidence vote and being replaced by Michael Howard.