No fewer than 45 per cent of tenants renewing their tenancies were given rent increases in August, the highest since Countrywide's rental index began.
The letting agency group - the largest in Britain - says the proportion of landlords increasing rents in London and south east England in particular rose to over 50 per cent.
Countrywide says the total number of homes advertised to let was down eight per cent in August compared to last year, whereas the number of registered applicants was up four per cent.
The average increase in rent for a renewing tenancy was 2.5 per cent - but this was still less than the annual increase in rent for newly let homes, which has hit 3.8 per cent.
The south west and Midlands saw newly let rents rise faster than anywhere else in the country with growth of 7.0 and 6.3 per cent respectively.
Growth in newly let rents was seen in all regions except in Central London where rents decreased 1.4 per cent year-on-year.
“Falling numbers of homes available to rent and increasing demand from tenants have been the defining features of rental market so far in 2015. Nine tenants are now registered for every home available to rent, up from 7.5 in August last year" says Countrywide's new research director, Johnny Morris.
“With pressure on rents and increased competition for homes on the market, the proportion of renewing tenancies seeing an increase in rent has grown. Faced with the choice of staying put or moving in a market with more competition and increasing rents, more tenants are choosing to accept a smaller increase in rent to extend their existing tenancy" he says.