Total sales were down 1.4 per cent, also a new low.
All sectors except food and footwear declined. Excluding seasonal fluctuations for Easter, it was the worst month on record.
The second half of the month was marginally better than the first, because consumers delayed spending for promotions.
"There are truly dreadul numbers," said BRC director general Stephen Robertson.
"Non-food retailers had a torrid December despite a blizzard of promotions and deals, which would have hit margins. Many hard-pressed consumers couldn't be seduced into spending."
Non-store non-food sales were the only bright spot, up 30 per cent as customers became more confident in delivery times.