The cost of the average private tenancy in Greater London is now £1,521 a month, double the £744 average for the rest of the UK.
That is a rise of 7.7% on the first three months of 2015, when the average London rate stood at £1,413.
The average UK rent outside Greater London was 4.8% higher than the same period last year when rents cost £710, according to the latest HomeLet Rental Index.
That is slightly lower than the 5.5% growth seen in the three months to January but rents on new tenancies continue to rise much faster than inflation in most parts of the country.
The proportion of single tenancies has fallen as more families become tenants.
Last year, single tenants accounted for just 33% of new tenancies on rental properties, down from 67% in 2008.
By contrast, the proportion of new tenancies signed by two tenants rose from 28% to 52% over the same period.
New tenancies signed by three or more tenants have risen from 5% to 15% of the market.
This trend may in part reflect the increasing number of families moving into the private rental sector as house prices have become less affordable.
Latest data from the Office for National Statistics reveals the number of privately rented homes let to families with dependent children has risen from 30% to 37% over the past 10 years.
Martin Totty, Barbon Insurance Group’s chief executive officer, said there is a continuing imbalance between demand and supply in the private rental market.
"The cost of a new tenancy continues to rise more quickly than general inflation.
“Landlords are letting out homes to many more families, with rental property representing an increasingly important alternative to owner occupation.
“We’re also seeing people manage with higher rents by meeting the costs as joint tenants.”