The legal framework for elections used to be based on the constituency, with anyone who paid a deposit and gathered a small number of signatures able to stand.
There was strict control of the amount that each candidate could spend on their election in the constituency but none over how much the parties could spend on the national campaign.
Campaign spending in Britain has been very low compared with the United States.
Concern at the influence of wealthy people giving money to the parties led to the Electoral Commission to introduce regulations.
Parties now have to register to be able to stand in elections and donations have to be declared.
Constituency spending has been relaxed and the upper limit is now about £40,000 between December and May but the national parties are limited to about £20m.
In 2010, the Conservative spent £17m, Labour £8m, much less than in 2005, and the Liberal Democrats £4m.
Spending in the 2015 General Election led to a court case. The issue was the distinction between national and local spending when Conservative Central Office was sending people to support local candidates.
