The total value of properties owned by buy-to-let landlords increased by £5.79 billion between November 2006 and November 2007, new research has found.

Figures compiled by Sainsbury''s Finance revealed that landlords in the UK have 2.76 million properties in their portfolios, with their total value climbing by an average of £5.79 million each month.

The biggest price increases were in London, with buy-to-let properties in the capital increasing in value by a total of £24.4 billion, while properties in Wales were the weakest performers, growing by just £942.72 million.

Steve Johnson, head of home insurance at Sainsbury''s Finance, said: "The buy-to-let market has grown substantially in recent years.

"People have invested a huge amount of money into this sector and as a result of rising house prices many have seen the value of this investment grow substantially."

As well as increasing the value of the property itself, house price growth has given a further boost to buy-to-let landlords by making it harder for first-time buyers to get onto the property ladder and forcing them to rent.

This has created strong demand for rental properties, helping improve rental yields.