Workers in Preston earn hundreds of pounds less than the average annual salary in the North West each year.
Income inequality charity the Equality Trust says the figures, which reveal a gap of more than £35,000 between the UK's richest and poorest areas, "paint a bleak picture for our society".
The average worker in Preston made £20,900 before tax in the 2016-17 financial year, HMRC figures show.
That's lower than the £21,300 median income across the North West.
HMRC uses the median, the middle number in a series, instead of the mean average, so the figures are not distorted by extreme highs and lows. The data only covers taxpayers, and does not include people who are self-employed.
Workers in Copeland have the highest median salary in the region at £26,000, while Blackpool employees have the lowest, at just £17,900.
Dr Wanda Wyporska, executive director of the Equality Trust, said: "These figures paint a bleak picture for our society, and we can see a huge divide between incomes in the north and the south [of the UK].
"These gaps are further evidence of the shockingly high level of inequality in the UK, which we know is linked to poverty, mental and physical ill-health, and lower levels of social mobility.
"This damages us all, rich and poor."
Across the UK, workers in the City of London have the highest income at £54,300, while employees in Boston, Lincolnshire, have the lowest, at just £17,600. The median is £23,600.
The Joseph Rowntree Foundation, a poverty and social mobility charity, urged the Government to focus on strengthening the economies of poorer areas in the UK.
Head of policy and partnerships Katie Schmuecker said: “We all want to live in a society where working hard means being able to improve life for your family, but too many working people are locked in poverty by low pay and high costs. That is not right.
“All parts of the UK need access to the jobs and skills that provide a route out of poverty, and incomes need to match the rising costs which leave many working families struggling to decide which bills they can pay.
"Until work provides a reliable route out of poverty, too many workers will struggle to make ends meet.”