Teachers in England and Scotland earned less in real terms in 2015 than a decade before, says an annual report from the Organisation for Economic Cooperation and Development (OECD).
This decline in income is in contrast to international trends, where average teachers' pay has been increasing.
The OECD's comparisons follow reports of plans by the UK government to lift public sector pay caps.
The National Audit Office says schools are struggling to appoint new teachers.
The international monitoring report on education, published each year by the OECD, shows that for teachers in England, with 15 years experience, pay had fallen by 12% between 2005 and 2015, when inflation had been taken into account.
For teachers in Scotland with similar experience, pay had fallen in value by 6% over the decade.
Austerity pay limits
But across developed countries, the OECD survey shows an upward trend for teachers' pay, up on average by 6% in primary and lower secondary and 4% for upper secondary.
The study also shows that teachers in England and Scotland - and on average across the OECD - earn less than other graduate-level workers.
There have been claims that ministers are going to relax the restraints on public sector workers' pay.
The cap on pay, initially of 0% and then 1%, has been in place since 2010, as part of austerity measures.
The pay review body for teachers announced in July that the 1% pay increase limit for teachers would continue for another year.
But the pay body, carrying out government pay policy, warned of a "real risk that schools will not be able to recruit and retain a workforce of high quality teachers to support pupil achievement".
Last week, Scotland's First Minister Nicola Sturgeon announced the 1% cap on public sector pay rises in Scotland would be scrapped next year.
There have been warnings from head teachers that salary levels are contributing to recruitment problems.
On Tuesday, the head of the National Audit Office, Amyas Morse, said ministers needed to "urgently address" the teacher shortage.
"Schools are facing real challenges in retaining and developing their teachers, with growing pupil numbers and tighter budgets."
A Department for Education (DfE) spokeswoman said more teachers were being recruited - with 15,500 more working in the classrooms than in 2010.
There were "significant sums" being spent on teacher recruitment, said the DfE.
"We recognise there are challenges facing schools and we are taking significant steps to address them."
The annual report also shows that the UK continues to spend more of its national wealth on education than any other developed country.
The OECD's annual report says the UK is the highest spender on education, with 6.6% of GDP.
This includes private and state spending, with tuition fees in England pushing up levels of private spending on education.