Despite auto-enrolment reaching 10m people, the policy is only reaching a third of the UK workforce, research has shown.
Aegon analysed the number of auto-enrolled savers in each constituency, based on data up to January 2019 made available by Guy Opperman, minister for pensions and financial inclusion.
It found London had the highest number of auto-enrolled savers, and the constituency of the Cities of London and Westminster provided the largest share of savers, with 600,000 individuals saving into a workplace pension.
Kate Smith, head of pensions at Aegon, said: "The figures show that the highest concentration of auto-enrolees is generally associated with city centre employers.
"However, while the total number of employees who are auto-enrolled may appear high and has now moved beyond the 10m milestone, this only represents around a third of the UK’s labour force.
"This means that a large proportion of the workforce are either self-employed, do not earn the £10,000 required to qualify for auto-enrolment or are under 22 years old."
Employees may have also opted out because they are entitled to a defined benefit pension or have other pension arrangements in place.
Number of employers per constituency who have declared compliance with their AE duties (2012 to 2019)
Number of eligible jobholders automatically enrolled per constituency (2012 to 2019)
Number of VAT and/or PAYE based businesses by constituency
Cities of London and Westminster
Holborn and St Pancras
Islington South and Finsbury
Milton Keynes North
Bermondsey and Old Southwark
Uxbridge and South Ruislip
In its 2017 auto-enrolment review, the government announced that it will be lowering the age for auto-enrolment from 22 to 18 years, and changing the way pension contributions are calculated, but this is tabled for mid-2020s.
In December, the Department for Work and Pensions announced that the minimum threshold for auto-enrolment would remain at £10,000 for 2019/20, which sparked criticism.
In the meantime, the government has explained that it intends to understand properly the impact of the 2018 and 2019 increases in minimum contribution rates before committing to a timetable for these changes.
Ms Smith added: "The UK’s employment rate is currently the highest since estimates began, and while rises to the state pension age will have a bearing on this, the growth of self-employed and gig-economy workers, for whom the benefits of auto-enrolments do not apply, is also a significant factor.
"It is crucial, therefore, that we find ways to make pensions more attractive to this large proportion of the workforce or they will end up with a significant shortfall in funds for retirement.
"The workplace pension forms the basis of many people’s retirement income and although it has been a successful catalyst for employees to begin saving, much more is needed to be done to help out those who are being left behind through ineligibility."
According to Martin Bamford, chartered financial planner and managing director of Informed Choice, auto-enrolment "has been a fantastic innovation to encourage greater pension savings".
He said: "It's not perfect, and there are big gaps to fill, but as a starting point it's important we don't underestimate the lasting impact this policy will have.
"It would be great to see even more employees, and especially the self-employed, encouraged in the future to save for their retirement.