The latest Coffer Peach Business Tracker shows a 1.2 per cent fall in like-for-like sales, with data drawn from 15 restaurant and pub chains including some of the biggest names in the sector such as Mitchells & Butlers, Whitbread and Tragus.
Total sales, which include those from new openings, were marginally ahead, up 0.3 per cent on May 2009.
Part of the reason for May's fall was put down to school half term falling in the first week of June.
Richard Hathaway, head of travel, leisure and tourism at KPMG, which co-produces the report, said: "It is no great surprise that trading conditions have continued to be tough and this year's figures show that overall there has been little change in sales compared with 2009, despite some individual monthly movements.
“Increasing sales is a challenge in such a competitive market, particularly with consumer confidence still fragile."
Tough year ahead
Month-on-month, May’s sales were down -18.2 per cent against April, which had the benefit of Easter.
The authors of the report reiterated their warning made last month that restaurant and pub operators face a tough year ahead.