Like-for-like sales fell 2.6% and total sales fell 0.4% in November according to the British Retail Consortium.
The BRC said it was the first time its sales monitor had shown two consecutive months of declining total sales. October sales showed a decline of 0.1%. The monitor began in 1995.
The BRC said the clothing sales declines got worse in November with sharp falls in sales of both menswear and womenswear although kidswear showed a small gain. Discounting and promotions largely failed to tempt shoppers over the month. Fashion sales have now been lower than year earlier levels for 13 of the past 14 months.
Footwear sales also fell back, recording their largest year-on-year decline since July. Both men's and women's footwear were well down on last year but kid's footwear held up.
Department store trading worsened further. Fashion sales in department stores slowed and premium ranges, which had previously held up, also weakened.
Internet and mail order sales also slowed although they were still up 9.5% in November. However this was against a rise of 16.6% in October.
BRC director general Stephen Robertson said: "The numbers speak for themselves - these are clearly tough times...Retailers will be hoping that customers have been putting off Christmas shopping - not cancelling it."
Robertson added: "With money tight at the moment shoppers can take advantage of the extraordinary levels of deals, offers and promotions. The recent VAT and interest rate cuts may also provide some stimulation."
Sharon Hardiman, head of non-store retailing (online, mail order and telesales) at the BRC said: "Non-store retail sales are clearly not immune to the economic slowdown. While they are still outstripping traditional in-store sales, November's performance slowed considerably compared to the previous month's double digit result."