The value of average earnings of full-time workers has fallen by 13.6% over the past seven years, according to a new study.
The biggest fall has been in London, where the real value of average pay has dipped by 23.2%, compared to 4.9% in Northern Ireland, a report found.
The next largest drop was 16% in the South East, followed by the East Midlands (15.7%), Yorkshire and the Humber (14.9%), North West (12.4%), West Midlands (11.8%), East (11.5%), Wales (11.3%) South West (10.6%), Scotland (6.7%) and North East (5%).
Local areas which have seen the biggest decline since April 2008 were in London, including Hammersmith and Fulham (52.6%), Islington (35.1%), Camden (29.3%), Southwark (28.3%) and Ealing (26.6%), research by the GMB union revealed.
The study of official figures showed mean gross annual pay was £31,494 in April 2008, rising to £33,689 in April 2015, a rise of 7%.
Paul Kenny, general secretary of the GMB, said: "While we have seen a growth in the number of workers as the population has grown, average pay has simply not kept pace with inflation.
"Since 2008 the cumulative inflation has been 20.6%. During this period, pay in the UK has gone up by 7%, which has left the pay of the average full time worker down by 13.6% in real terms.
"This has had a deflationary impact on the economy and has also affected the tax take by the Chancellor to pay for essential public services.
"In the autumn statement, the Chancellor predicted that the economy would grow steadily each year to 2020 when it would be 12% bigger than now.
"Workers in the UK will want to see that growth translating into pay rises above inflation to make up for lost ground."
A Treasury spokesman said: "We have record levels of employment, wages are up 2.3 per cent above inflation in the past year, and the new National Living Wage will deliver a further boost next year."