Extracts from her upcoming book reveal that Angela Merkel viewed Brexit as a “humiliation” for the remaining EU members.

The former German Chancellor played a mediating role in the years leading up to Brexit, using her influence to try and help David Cameron obtain the concessions he believed he needed to convince his party and the British people that the country’s future was within the union.

She writes: “I tried wherever possible to help David Cameron. He insisted on a seven-year EU budget, which would at the very least show no growth in comparison to the previous one, but which included plans for higher expenditure on research and innovation.

“This was an attack on all the recipient countries who received more money from the EU budget than they contributed, and who were now going to have less money at their disposal for their economic development.”

Merkel believes that the referendum’s roots date all the way back to 2005, when a young David Cameron promised as part of his Conservative leadership campaign to “leave the European People’s Party because it was too pro-EU.”

She said: “Seven years later, in January 2013, he announced in a speech that if he were to win the next general election and be re-elected as prime minister, a referendum would be held on the UK’s membership of the EU during the first half of the subsequent parliamentary session.

“He himself was in favour of remaining, but he was attempting to gain the votes of the sceptics.

“With this promise he succeeded in emerging victorious from the election on May 7, 2015, but his rivals within the party wouldn’t let it rest; on the contrary.”

The former Chancellor believes that the intervention of Boris Johnson, who eventually pledged his support and political weight behind the leave campaign was crucial in deciding the outcome of the election.

She said: “Boris Johnson, one of his most influential competitors in the power struggle for leadership of the Conservative Party, decided, against Cameron’s hope, to support the UK’s exit from the EU in the 2016 referendum campaign.

“This gave the EU opponents’ campaign the decisive boost. They prevailed. “

Ms Merkel reveals the torment she faced over whether she could have done more to prevent it.

She reflects: “I came to the conclusion that, in the face of the political developments taking place at the time within the country, there wouldn’t have been any reasonable way of my preventing the UK’s path out of the European Union as an outsider.

“Even with the best political will, mistakes of the past could not be undone — this was a bitter lesson.

“And so all that remained for me was the hope that the UK and the EU would maintain a sense of mutual appreciation, and that when it came to important matters, they would find ways and means of engaging in dialogue that led to harmonious agreement.”