United States
In the United States, there is a clear line of succession should the President die or be incapacitated. A number of constitutional mechanisms and congressional laws exist to ensure the continuity of government. These include:
Article II, Section 1, Clause 6: Article II of the Constitution stipulates that the vice-president is first in the line of succession if the president is ‘disabled or removed or has died’.
12th Amendment: This constitutional amendment stipulates that the House of Representatives will fill any vacancy in the presidency if the election fails to return an absolute majority for any candidate.
25th Amendment: This amendment was passed in 1967 to clear up some uncertainties in the line of succession. For example, under Article II, Section 1, it was unclear whether, if it was invoked, the Vice-President became President, or merely assumed the power of the President. The 25th Amendment dealt with a variety of issues:
Section 1 – This clarified that the Vice-President would become President if the President died or resigned. This is why, for example, Gerald Ford became President hen Richard Nixon became the only President to resign in August 1974.

Section 2 – If there is a vacancy for Vice-President (for example, when Gerald Ford assumed the Presidency), the President would nominate a Vice-President to be confirmed by both Houses of Congress. Prior to this, the Vice-Presidency would remain vacant if the Vice-President assumed the Presidency. For example, when John F. Kennedy was assassinated in November 1963 and Lyndon B. Johnson became President, the Vice-Presidency remained vacant until the 1964 Presidential Election.
Section 3 – This allows the president to voluntarily transfer his powers to the Vice-President. This was invoked twice by George W. Bush when he underwent operations and temporarily handed power to his Vice-President, Dick Cheney.
Section 4 – This section allows for the removal of the President if a majority of the Cabinet vote in favour of such a move. The removal must be because they believe the President is ‘unable to discharge the powers of his office’. In 1987 it was rumoured that the cabinet of Ronald Reagan considered invoking section 4 following concerns about his work ethic and a belief that he was losing some of his mental faculties.

The constitution also provides for Congress to provide a ‘line of succession’ for the Presidency. The latest act was the Presidential Succession Act (1947) which created the line as it stands today:
Vice-PresidentSpeaker of the House of Representatives
President Pro Tempore of the Senate
Secretary of State
Secretary of the Treasury
Secretary of Defense
Attorney General
Secretary of the Interior
Secretary of Agriculture
Secretary of Commerce
Secretary of Labor
Secretary of Health and Human Services
Secretary of Housing and Urban Development
Secretary of Transportation
Secretary of Energy
Secretary of Education
Secretary of Veterans Affairs
Secretary of Homeland Security
Famously, at the State of the Union Address, when the Cabinet gather to hear the President address Congress, one member is nominated to be the ‘Designated Survivor’. This is to ensure there is continuity of government should a disaster strike. This is the premise of the Netflix Series of the same name.
In the United States there is a clear line of succession. Since the passing of the 25th Amendment in 1967, it is pretty unambiguous and gives clarity as to who will lead the government if the President is in someway incapacitated.
Britain
The ‘line of succession’ is far less clear in the UK. As a system that relies heavily on uncodfifed constitutions, as opposed to a codified constitution, this perhaps unsurprising.
The lack of a formal line of succession has been bought into focus recently following Boris Johnson’s stay in Intensive Care, during which he has readily admitted that at times it was 50-50 whether he would make it out.
Standing in form him during his period in intensive care and recovery was Dominic Raab, the Foreign Secretary. Raab took over the powers of the Prime Minister as a result of being First Secretary of State.

The position of First Secretary is not a formal constitutional role. It is instead awarded by the Prime Minister to the minister that wishes to the de facto second person in government. In recent years this title has been bestowed on Damian Green (under Theresa May), George Osborne and William Hague (both under David Cameron). Notably, in the absence of the Prime Minister, the First Secretary of State usually covers PMQs in the Prime Minister’s absence, as William Hague did famously against Harriet Harman:
Sometimes, the position of Deputy Prime Minister has been created in the UK. This last happened between 2010-2015 when Nick Clegg was Deputy Prime Minister in the Coalition Government. However, despite the largely honorific title, Clegg would not have taken over from David Cameron if he was seriously incapacitated. Instead, William Hague or George Osborne, the First Secretaries of State, would have deputised until the Conservative Party could pick a new leader.

The Queen visited Harold MacMillan in hospital. During this meeting, MacMillan advised the Queen to appoint Alec Douglas-Home as Prime Minister and not Butler. MacMillan was known to despite Butler and did not want him as his successor. Following the advice of the Prime Minister, the Queen asked Alec Douglas-Home to form a government. This was controversial and led to accusations that the Queen was acting beyond her constitutional role. The matter was further complicated by it being widely known that the Queen herself was personal friends with Douglas-Home.
One of the potential disadvantages of the British system, as compared to the US, is the lack of a democratic mandate that a successor may have. Unlike in the US, where members of the Cabinet have to be confirmed by the Senate, the appointment of the Cabinet in the UK is solely a prerogative power. So, in the case of the First Secretary of State, like all other cabinet members, they are in place without a wider democratic mandate and may act with all the powers of the Prime Minister.
Of course, in Britain, the Prime Minister themselves is indirectly voted, so the same criticism could be levelled at them. However, generally, the Prime Minister receives a democratic mandate from the people in a General Election, as Boris Johnson did in December 2019.
So, should Britain codify the line of succession? At present, it seems unnecessary. The Douglas-Home situation is no longer going to emerge in the age of Party Leadership elections and the COVID crisis itself may have made the current convention more transparent to the British public for the future.