Buckingham Palace was infuriated with Theresa May after she misled the Queen about the progress of negotiations as she sought to form a government, it has been reported.

Tensions are said to have arisen between senior courtiers and Downing Street after the Prime Minister said she had secured the support of the Democratic Unionist Party (DUP), only for it to take another 17 days before a deal was finalised.

The palace was further irritated by Ms May breaching protocol in the words she used to announce her intention to form a government on the steps of Downing Street, The Sunday Times reported.

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A Tory peer told the newspaper: “She said, ‘I’ve formed a government’, not ‘The Queen has asked me to form a government’. The palace was alarmed by her formulation of words.”

The affair did not prompt a formal complaint from the monarch but the Queen’s private secretary, Sir Christopher Geidt, is said to have repeatedly made his frustrations clear.

“It is certainly true the palace was being given a greater sense of certainty about the deal than was accurate, because the truth is that the DUP buggered Mrs May about longer than she thought,” said a royal source.

The palace grew increasingly frustrated at Ms May’s “lack of courtesy”, because the delayed state opening of Parliament disrupted plans for Royal Ascot, they added.

A Conservative adviser, however, said Ms May did not intentionally misled the Queen, because her chief whip, Gavin Williamson, “thought he had assurances he didn’t have” from the DUP.