BBC News Online explains the facts behind the issue. NI was introduced in 1948, as a key part of the welfare state, to ensure that people had protection against poverty when they became old or sick. However, the value of many of the benefits provided has been reduced over time.
Employers also pay NI contributions, complementing the contributions levied on their employees.
That is more than it raised from VAT, and more than half the amount raised from income tax. Like income tax, the amount collected by the government has risen sharply in recent years, helping to boost the government's budget surplus. The Conservatives say that Labour has a secret agenda to raise NI contribution levels, either by raising the rate of tax or by extending the upper limit at which NI is payable. They claim that if Labour abolished the upper earnings limit, it would affect some four million taxpayers, costing them 800 each. The Conservatives argue that Labour needs to raise more taxes to fund its plans to expand public services in the last two years of the next Parliament.