LONDON, ENGLAND - JUNE 21:  Queen Elizabeth II  walks up the staircase in 10 Downing Stree with Prime Minister David Cameron before having lunch together to mark the Duke of Edinburgh's 90th birthdayon June 21, 2011 in London, England. The Camerons have invited the Duke of Edinburgh and The Queen to Downing Street to celebrate The Duke's 90th birthday.  (Photo by Stefan Rosseau - WPA Pool/Getty Images)

Queen Elizabeth II's 12 prime ministers

Updated 2:51 PM ET, Thu July 14, 2016

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London (CNN)During her 64-year reign, Queen Elizabeth II has been served by 12 prime ministers. Many relationships were formed -- from Winston Churchill, to "Iron Lady" Margaret Thatcher -- and some proved more difficult than others.

While most of their political and personal conversations are kept strictly confidential, memoirs and historic interactions provide an insight into their relationships.
So as Theresa May becomes Britain's next Prime Minister -- and Her Majesty's 13th -- take a look back at those who have served her.

WINSTON CHURCHILL 1951-1955

Princess Elizabeth greeting Winston Churchill At Guildhall on March 23, 1950.
The Queen was said to be in awe of her first prime minister, Winston Churchill. Once when asked which PM she enjoyed meeting with most, she replied: "Winston of course, because it's always such fun."

ANTHONY EDEN 1955-1957

The Queen shaking hands with the Conservative British Prime Minister, Sir Anthony Eden, in May 1956.