Senior official in the British Civil Service
The Principal Private Secretary to the Prime Minister of the United Kingdom is a senior official in the United Kingdom Civil Service who acts as principal private secretary to the prime minister of the United Kingdom. The holder of this office is traditionally the head of the Prime Minister's Office in 10 Downing Street. In the Civil Service, the role is currently graded as director general. 
The current principal private secretary is Nin Pandit.
History.
As the role of Prime Minister has evolved over time, so has the role of Principal Private Secretary. The Duke of Newcastle appointed a Private Secretary during his second term of office as First Lord of the Treasury (1757-62), and his 18th-century successors for the most part did likewise. The Private Secretary at this time was not on the establishment of HM Treasury and he was not paid an official salary. This changed in 1806, when money was made available from public funds. In 1813 the funding available was doubled to enable a second Private Secretary to be employed, alongside the first (or 'principal') holder of the office. 
At this time it became customary for one of the Private Secretaries to be appointed from within the Civil Service (more often than not from the Treasury) to provide administrative support, and the other to be recruited from outside the Civil Service (including on occasion from the House of Commons) to provide political support. (In today's terms the latter were more akin to special advisers or a Parliamentary Private Secretary.) A third Private Secretary was added in the 1870s.
Under Lloyd George (Prime Minister from 1916 to 1922), the duties of the Principal Private Secretary changed, as some responsibilities were transferred to a new Secretariat (the so-called 'Garden Suburb'), while others (particularly those relating to meetings of the Cabinet and the implementation of its decisions) were taken up by the newly established Cabinet Office and Cabinet Secretary. At the same time the number of clerical staff working in the office was substantially increased, which eased the administrative burden. The Principal Private Secretary was left to focus on arranging the Prime Minister's diary, offering advice and personal support, and overseeing certain particular areas (such as honours and appointments).
Up until the 1920s the private office of the Prime Minister was usually staffed by personal and/or political appointees; a new Prime Minister would often bring in his own people (either from the civil service or from outside), and there was invariably a change of staff with a change of government. Ronald Waterhouse and Patrick Gower, however, who were appointed to the office by the Conservative Bonar Law in 1922, remained in post not only under his Conservative successor Stanley Baldwin but also under the first Labour Prime Minister Ramsay MacDonald. Since then the Principal Private Secretary has been considered a member of the permanent Civil Service, rather than a political or personal appointee, and as such may remain in office in spite of any change of administration.
For many years, although there was a hierarchy of officials within the private office, it was not generally made public (instead the 'private secretaries' were simply listed in alphabetical order); but this changed in 1929 when Robert Vansittart was listed as 'principal' private secretary for the first time in the "Imperial Calendar". 
Recent history.
During Tony Blair's administration, the prime minister (as Minister for the Civil Service) modified the law under which the Civil Service operated (through an Order in Council) which gave power to the newly created role of Downing Street Chief of Staff (a politically appointed special adviser) to give instructions to civil servants and outranked the principal private secretary in the Downing Street power structure. When Gordon Brown entered 10 Downing Street, he reversed the change to the Civil Service law.
When David Cameron became prime minister, he promoted his then principal private secretary to a new post of "Downing Street Permanent Secretary"; a position which took over as the top civil servant in the Prime Minister's Office and for the first time the head of the office held the highest rank in the UK's civil service. In 2012, when the post-holder, Jeremy Heywood, was appointed Cabinet Secretary; this new post ceased to exist, and the chief Civil Service official in 10 Downing Street reverted to being the Principal Private Secretary, which remains to this day.
The principal secretary runs the private office of the prime minister, which includes the Private Secretary for Foreign Affairs to the Prime Minister.