•NHS funded hip and knee replacement admissions doubled and trebled from 1997/8.•NHS admission rates have fallen since 2007/8.•Private sector admission rates have increased since 2003/4.•Inequality has grown since 2003/4.•Inequality has increased fastest in the private sector since 2007/8. Parliament has imposed duties on the government and NHS in England aimed at reducing health inequalities. to understand the effect on inequalities of government policies, which require the NHS in England to outsource elective surgery to the private sector. We analysed the numbers of admissions for hip and knee replacement surgery from the least and most deprived population quintiles in three time periods: before the introduction of the policies (1997/98–2002/03); following the implementation of the independent sector treatment centre programme (2003/04–2006/07); and after the extension of 'choice at referral’ (2007/08–2018/19). despite admission rates doubling and trebling for hip and knee replacements, respectively, between 1997/98 and 2018/19, inequality grew to the detriment of the most deprived. Inequality grew at the fastest rate during period 3; admission rates to the NHS fell while admissions to the private sector continued to rise. By 2018/19 almost a third of NHS funded procedures were provided privately. In 1997/98, for every 10 patients admitted for hip and knee surgery from the most deprived quintile, 13 and 9, respectively were admitted from the least deprived, by 2018/19 the gap had widened to 19 and 15, respectively. Socio-economic inequalities for hip and knee replacement have widened as outsourcing of NHS treatment to the private sector has increased. The NHS must rebuild in-house capacity and provision instead of outsourcing care.