In October 2007 the Treasury published a report Income tax and National Insurance alignment: an evidence-based assessment. It is available at:
The report noted that changing to a National Insurance system in which earnings were cumulated across all employment for those with more than one job would allow a small number of individuals (currently estimated at around 40,000) to gain access to contributory benefits. However others, including some low earners, would lose out by having to pay more National Insurance Contributions. The report considered more widely aligning the administration of National Insurance Contributions with that of income tax. The Government's conclusion in the Report was that on balance the benefits of administrative alignment did not outweigh the costs.