Some 20% of women, who often take career breaks or work part time to support families, said they have no other pension provision, compared with 7% of men, the research by insurer Prudential found.
The research, carried out among more than 1000 people intending to retire across Britain this year, also found that 18% will have an income below the "minimum income standard" as defined by the Joseph Rowntree Foundation (JRF).
The JRF estimates that a single pensioner needs an income of at least £8600 a year to reach a minimum socially acceptable standard of living; a retired couple needs an annual income of more than £12,500.
In Scotland, 9% of people retiring this year said they will be doing so without any private pension, which was the second lowest figure after the North West. In Wales this figure was 16%.
The state pension typically makes up 42% of a woman's expected retirement incomes compared with 28% for a man.
The findings follow shake-ups announced by the Government for the pensions industry in recent weeks.
Vince Smith-Hughes, a retirement income expert at Prudential, said these would give retirees choices, but added: "They don't alter the fact that many people are not saving enough for a comfortable retirement."
Minister for Pensions Steve Webb said women have been treated as "second class citizens" in retirement for too long. He said: "We are building a fairer retirement by automatically enrolling millions of workers and introducing a new flat-rate state pension."