It does not entail any specific legal responsibilities, though the holder may be assigned some, and is usually paired with a departmental secretary of state position.

[3] Deputy prime minister is a title[4][5][6] and successive monarchs have refused to officially recognise the position.

[10] When Winston Churchill attempted to have Anthony Eden appointed deputy prime minister in 1942, George VI said that the 'office ... does not exist' and that conferring the title may be seen as an attempt to designate the prime minister's successor and thus may restrict the monarch's royal prerogative.

[3] Rodney Brazier has written that there are three reasons why a deputy prime minister has been appointed: to set out the line of succession to the premiership preferred by the prime minister, to promote the efficient discharge of government business and (in the case of Labour governments) to accord recognition to the status of the deputy leader of the Labour party.

[13] Jonathan Kirkup and Steven Thornton suggest that there are multiple motivations behind a prime minister appointing a deputy: leader of a party in a coalition government, as their designated successor, to neuter or mollify a rival, because they are a 'safe pair of hands' and to create a 'balanced ticket'.

[15] Bogdanor, Brazier and Anthony Seldon also suggest that the title may be of use if a prime minister were to die or fall unable to exercise their functions.

This designation was seen as an exceptional result of a coalition and the war,[19] and Attlee's 1942 appointment was not formally approved by the King[20][21][22] and was a matter of form rather than fact.

[7] The designation was because Churchill wanted to demonstrate the importance of the Labour party in the coalition, not for any reasons relating to succession; he actually left written advice that the King should send for Anthony Eden if he were to die, not Attlee.

[23] As the title of deputy prime minister did not hold any statutory authority, Heseltine was also appointed as First Secretary of State.

During the coalition William Hague was appointed by Cameron as First Secretary of State, the only time that both these positions have existed concurrently but not been held by the same person.

[19] It has also been suggested that the office of Lord President of the Council (which comes with leading precedence) has been intermittently used for deputies in the past.

[43] And, on 6 April 2020, when Prime Minister Boris Johnson was admitted into ICU, he asked First Secretary of State Dominic Raab "to deputise for him where necessary".