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.
Her Majesty found her second prime minister to be a sympathetic listener and their relationship was one of constitutional propriety. The largest political event to occur during Eden's time was the Suez crisis. During this time, he believed it was of supreme importance to keep the Queen informed, so he shared all of the Suez papers with her -- the first time she had ever been shown secret government documents.

    HAROLD MACMILLAN 1957-1963

    Harold Macmillan gives a speech at the inauguration ceremony of a memorial to John F Kennedy.
    The Queen originally found Macmillan difficult to deal with, but they eventually warmed to each other. Her Majesty relied on Macmillan for his wise counsel -- both while in office and after his retirement in 1963.

    ALEC DOUGLAS-HOME 1963-1964

    The Queen meets former Prime Minister Sir Alec Douglas-Home and Lady Home at County Hall in London, November, 11 1964.
    The Queen was well acquainted with Douglas-Home as he had been a childhood friend of the Queen Mother. So Her Majesty worked hard to re-establish her informal relationship with him. Over the year he was in office, Douglas-Home helped the monarch name several royal horses.

    HAROLD WILSON 1964-1970, 1974-1976

    British Prime Minister Harold Wilson shaking hands with the Queen outside 10 Downing Street, following his resignation, London, March 24, 1976.
    Wilson, who came from a lower-middle-class background, became the Queen's first Labour Party prime minister. Wilson often broke away from meeting traditions, and enjoyed helping with the washing-up after barbecues at Balmoral -- one of the Queen's residences. The Queen, however, warmed to Wilson's informal presence and even invited him to stay for drinks after their first meeting, which was not commonplace.

    EDWARD HEATH 1970-1974

    The Queen with British Prime Minister Edward Heath at Heathrow Airport, London, January 28, 1974.