Retail sales in central London during February rose 10.5% like for like, boosted by sunnier weather than February 2007.
London continued to outperform the UK as a whole, where like-for-like sales growth fell to 1.5%.
Retail footfall rose above its year-earlier level for the first time since October. However, the gain was against a sharp fall last February when snow storms kept shoppers at home.
However, February's clothing and footwear sales remained generally difficult, but accessories and upmarket fashions held up.
BRC director general Stephen Robertson said: "These figures suggest the best like for like growth since October. An early Mothering Sunday and improved city bonuses helped retailers but there has been no let-up in the need for price cuts and promotions to tempt hard-up customers to spend."
KPMG head of retail Helen Dickinson added: "The like-for-like sales for central London continue to outperform the remainder of the UK, but also show a high level of volatility. This is driven by high levels of promotional, discounting and clearance activity, as well as significant variations in the performance of individual retailers."