Rent prices are now rising in every region of the UK, while the pace of growth in London has picked up again.
This is only the third time the HomeLet Rental Index has shown rent rises in all 12 regions of the UK.
It has only happened twice previously in the history of the index: once in October 2014 and once in December 2010.
Rents agreed on new tenancies across the UK in the three months to the end of May 2015 were 10.7% higher than the same time last year.
The average rent now stands at £935 per month, although this falls to £738 excluding Greater London.
In the capital, the average rent on new tenancies signed during May 2015 was £1,506 per month. That is the first time it has ever exceeded £1,500 a month.
Martin Totty, chief executive officer, Barbon Insurance Group, parent company of HomeLet, said: "Rental values are now increasing year on year across the country, with no exception.
"After a short period of London rents rising more slowly, when it seemed the rest of the UK may catch up or even exceed the capital in the speed at which rent prices were increasing, we now see the rate of price rises in London returning towards double digit growth, while the rest of the UK continues to rise steadily."
"The South West of England saw the largest increases, with average rent prices 13.6% higher than a year ago. Scotland (9.6%), the South East of England (9.4%) and Greater London (9.2%) also advanced strongly.
Totty continued: "With the whole of the UK experiencing increases in rent prices agreed on new tenancies, it is possible this is an early indicator of a post-election private rental market where both landlords and tenants might expect rent prices to keep rising as demand continues to grow."
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