The Home Office publishes quarterly statistics on applications to the Worker Registration Scheme. Figures published in August 2007 indicated that some 656,395 persons were accepted on to the scheme between 1 May 2004 and 30 June 2007, of whom 430,395 were Polish nationals. However, as the scheme is voluntary, offers no financial incentive and is not enforced; immigrants are free to choose whether or not to participate. They may work legally in the UK provided they have a Polish identity card or passport and a UK National Insurance number. This has led to some estimates of Polish nationals in the UK being much higher. Department of Work and Pensions (DWP) publishes quarterly reports containing data on National Insurance number (NINo) allocations to adult overseas nationals entering the UK. The number of Polish nationals’ NINo registrations peaked between 2006-2008. In the financial year 2006/07 there were 220,430 Polish nationals receiving NINo registration (31% of all NINo registrations to adult overseas nationals entering the UK) and in 2007/2008- 210,660 (29% of all registrations to adult overseas nationals). The number of NINo registrations granted to Polish citizens has been in significant decline since 2016 referendum. In the year to June 2016 Polish born adults received 105 thousand NINo’s, 18% less than in the year before a 13% share of all NINo registrations to adult overseas nationals entering the UK. The latest statistical data covering the year to the end of March 2020 shows a further decrease in Polish NINo registrations. During this period 38 thousand Polish citizens received NINos - 13% less than in the previous year and a significantly smaller share of all adult overseas registrations compared with previous years - 5%.