‘PM’s biggest fear is letting people down’ - Samantha

Mrs Cameron suggested he was the best man for the job of running the country because he was “even-tempered, clear-headed, and not scared of making hard decisions”, in the latest in a series of interventions by the spouses of party leaders.

Interviewed for the Mail on Sunday’s You magazine, she also gave an insight into some more unusual aspects of life as the UK’s “first lady” including being forced to leave dinner with Angela Merkel to break up a “huge pillow and duvet fight” between her children at the German chancellor’s country residence.

Once described by former Tory spin doctor Andy Coulson as the Prime Minister’s “best weapon”, Mrs Cameron - a consultant for luxury handbag and stationery firm Smythson - recently spoke of “desperately” wanting her husband to triumph.

“He’s turned the economy round and created a safe environment for everyone to live their lives. The thing that worries him most is letting people down,” she told the Mail on Sunday.

She paid an emotional tribute to the way her “amazing, strong and steady” husband had helped get them through the trauma of their first child Ivan’s severe disability and death, aged six, in 2009.

His “glass half full” approach had been vital when they were “close to breaking point”, she said.

The Prime Minister, in a separate interview with the Sunday Times, put their ability not to fall apart entirely down to his wife’s fortitude.

He said his marriage was “easily the best thing that’s happened in my life” and said he loved his wife “as much today as when I first met her; more, much more”.

He added: “I’m very blessed to have her. She is amazing.”

She admitted that sometimes the manners of the couple’s three children - Elwen, Nancy and Florence - were not always “what you might wish for”.

Speaking about the 2013 incident at Merkel’s “schloss” near Berlin, she said: “No one could get them to calm down, so there were some very firm words from me. When you take them out of the family context, you realise maybe their manners aren’t quite what you might wish,”

Nancy, 11, regularly joked about penning a memoir about life as the daughter of a PM - including one notorious incident, she revealed.