Residential rents for new lets in London have fallen for the first time in six years, according to the first study published on the issue since Britain voted to leave the European Union.

A monthly survey published by Countrywide estate agents on Monday found that new rental contracts were priced 0.5 percent lower in July than for the same period last year.

The average monthly rental contract in July was £1,280 ($1,647, 1,470 euros), around £7 cheaper than a year ago.

"The large rise in numbers of homes available to rent has certainly slowed rental growth, even with tenant numbers increasing," said Johnny Morris, research director at Countrywide.

He highlighted the increase in stock levels due to buy-to-rent investors bringing forward house purchases before new tax laws kicked in in April.

"Added to that, uncertainty in the sales market in the run up to, and after the EU referendum has caused more discretionary sellers to turn to the rental market," he added.

Nationally, rents for new contracts increased by 1.5 percent last year.

Britain's decision to leave the EU has led to predictions of a slowdown in the property sector, particularly in the booming London market.