When the United Kingdom came into existence, on 1 January 1801, the era of disciplined mass parties had not yet begun.
Most of these loose associations of politicians, after the disappearance of almost any party bonds by about 1760 and the accession of George III, contained members from both Whig and Tory traditions.
In the first decade of the 19th century most politicians realigned themselves into fairly cohesive Whig and Tory parties.
Thereafter individuals and groups might move between the two parties, but they both maintained a continuous existence (through a number of mergers and name changes).
Charles James Fox and Viscount Howick, as unofficial leaders of the party in the House of Commons from 1801 to 1807, led the largest of the anti-Pittite Whig groups.