More than half of the 20 Premier League clubs reported an operating profit during the 2007-08 season as the league's collective revenue increased to nearly £2bn, according to new research.

Deloitte's Annual Review of Football Finance revealed that Premier League clubs' revenues had increased by 26 per cent compared with 2006-07, and are now expected to break through the £2bn barrier during the 2008-09 season. However, the research also revealed that the economic downturn is having an impact on the amount of investors willing to inject cash into clubs as the league's net debt has increased to more than £3bn.

Two thirds of the total net debt has been attributed to the Premier League's top four clubs - Arsenal, Chelsea, Liverpool and Manchester United - while the cost of players' wages has also contributed. Salary expenditure reached £1.2bn following the largest annual increase recorded by the Premier League. Dan Jones, partner in Deloitte's Sports Business Group, said: "The domestic and international popularity of the Premier League continues to generate remarkable revenue growth. Between 1992 and 2008, revenues for the top 20 clubs grew at a compound annual rate of 16 per cent, compared with 5.4 per cent for the UK economy as a whole.

"The new economic realities may lead to flat matchday revenues. While attendances continue to hold up well, many clubs have frozen or reduced ticket prices. However, the stepped increases in the current domestic broadcast deal and the new UEFA Champions League TV deal make it likely overall revenues will edge up."