The US sales day that follows the Thanksgiving holiday brought the company its biggest trading week on record, with sales up 22% on last year.
However, Andy Street said the US phenomenon was “more challenging profitability-wise.”
“We’ve got to ask if it’s right to concentrate trade so much in that one period,” he told the BBC.
Overall, in the five weeks to 27 December, total like-for-like sales rose 4.8% to £777m, with shop sales flat, but online purchases up 19%. Click-and-collect represented 56% of online sales, overtaking home delivery, the firm said in a trading update.