This page summarises the rules that applied when the Worker Registration Scheme still operated. It ended on 30th April 2011.
The Worker Registration Scheme (WRS) was set up in 2004 when the eight accession countries joined the EU. It ran for five years initially and was then extended for a further two years and ended on 30th April 2011.
Accession state nationals could come to the UK and look for work freely, but once they got a job they had to apply to register on the WRS within one month of starting work. This was done on a specific UKBA form. Employers had to provide written proof of employment and the worker provided proof of identity and paid £90. The UKBA then sent back a Registration Certificate and a Registration Card. The card included the applicant's name, nationality, date of birth, registration number and photograph. The certificate was issued for that job and lasted for 12 months.
If the worker moved jobs, s/he had to apply for another certificate for the new job, but kept the same Registration Card. Once s/he had worked for 12 months continuously, s/he was allowed to work in the UK on the same terms as any other EU national and no longer needed to register on the scheme. S/he could then apply for an EEA residence permit (which other EEA workers can get as soon as they start work in the UK), which confirms this. When assessing whether the worker had worked for 12 months continuously, breaks totalling less than 30 days within this 12 month period were ignored, but if the worker was unemployed for more than 30 days within the 12 months, s/he had to 'start again' on the scheme, and pay the £90 again. About a third of those registered appear to have done this at least once.
If a worker was employed by an agency, s/he did not have to re-register if simply allocated different work by the agency.
The UK rules said that the ex-worker lost their right to reside in the UK, and so was not entitled to income-based benefits, etc. They may have been able to claim contributions-based benefits. These rules, however, have been challenged. The AIRE Centre complained to the European Commission that these restrictions were unlawful because they discriminated against some workers, and the commission wrote to the UK government to raise the issue. There are also cases on some aspects of these pending in the UK courts or on their way to the European Court of Justice. So if you have a case, it is important to contact the AIRE centre to discuss how it might be taken forward. A factsheet (and another which covers the issue of accession state and Bulgarian and Romanian nationals who are rough sleeping) is available from Migrants' Rights Network.
A practical approach was to explore the options for self-employment and employment at this stage, since there are no restrictions on accession state citizens going into self-employment in the UK and in practice any accession state national can work from 1st April 2011 without needing to register and be eligible for housing and benefits.
What happened to accession state nationals married or partnered to UK citizens or people with indefinite leave?
Before 1st May 2011, the accession state national had freedom of movement, but if s/he had not acquired other rights (through work, study or self-employment), then if the relationship broke down s/he was likely to have no access to benefits or housing because s/he stopped being self-sufficient. There were not even any arrangements in place to deal with cases of domestic violence. Cases like these need expert legal advice to mount a possible challenge to UK law and practice: contact the AIRE Centre.
An exception to this is the situation where a UK national had worked in another EEA country and then returned to the UK. S/he was regarded as 'bringing his/her EEA worker rights back to the UK with him/her': these cases are known as Surinder Singh cases after the UK case that established this ( 1 FCR 453,  Imm AR 565). These rights extend to family members, so if the relationship broke down, the spouse/partner/children usually kept the ex-partner's workers' rights (see EEA family members) and so retained the right to reside and eligibility for housing and benefits.