The average advertised salary in June 2013 was £33,414 - 2.1% lower than a year ago. This equates to a drop of more than £700 over the year. Factoring in inflation, wages are down £1,670.
The UK Job Market Report, from job site Adzuna, found London is the only region in which salaries increased - rising by 1.2% over the last six months. In Wales the average salary has fallen by 8.5% since December 2012.
As well as the fall in wages, the report found competition for jobs in Britain has also fallen for five consecutive months. The number of jobseekers per vacancy has fallen 3.3% in February 2013, to 2.9% in June 2013. It now stands at its lowest level since January 2009.
The number of advertised vacancies rose to 504,913, 2.5% higher than in June 2012.
Regional disparity
The report found six of the 10 areas with the lowest ratio of jobseekers to vacancies were in the South East. Oxford, Reading, Winchester and Guildford all have less than one jobseeker per vacancy.
However, Scotland saw a 6.7% fall in advertised job vacancies, which has resulted in 6.4 jobseekers for every available role.
Flora Lowther, head of research at Adzuna, said: "The labour market may appear to be making strides towards improved health, but underneath the surface lie several areas of concern.
"The narrowing north-south divide in job creation does not disguise the regional disparity with new vacancies skewed disproportionately towards the South East."
She added: "In Wales, although competition has eased since December 2012, the average salary on offer has fallen by £2,300 p.a. over the same period, although with inflation running at 2.9% the real-term fall in wages is well above this figure.
"Lower competition may attract further jobseekers to the region, but the drop in wages could well send them running to other parts of the UK where there are more jobs on offer, and where wage growth is far more positive."
Earlier this month ONS figures showed unemployment has fallen by 57,000 to 2.51 million in the three months to the end of May.
