The group, which sells everything from homewares and toys to electricals and food, said like-for-like sales in its main B&M UK business soared 23% in the 26 weeks to Sept. 26.
B&M's stores have been allowed to stay open through UK lockdowns because they sell some food.
"Our combination of everyday value across a broad range of product categories and convenient out-of-town locations has proved popular with shoppers," said Chief Executive Simon Arora.
Sales growth at B&M UK was forecast to moderate over the second half, but had so far been at a similar level to the first half.
Shares in the group have increased 43% over the last year, giving it a market capitalisation of 5 billion pounds ($6.6 billion) - more than that of both Sainsbury's and Morrisons, Britain's second and fourth biggest supermarket groups by sales.
The group, whose full name is B&M European Value Retail, made adjusted earnings before interest, tax, depreciation and amortisation (EBITDA) of 295.6 million pounds ($389.6 million) in the six months to Sept. 26, up from 151.4 million pounds in the same period last year. Revenue increased 25.3% to 2.24 billion pounds.
B&M has expanded rapidly. It has 963 stores in the UK, trading as B&M, Heron Foods and B&M Express, and 103 stores in France, trading as Babou and B&M.
It plans 40-50 B&M UK gross openings in the full 2020-21 year, offset by 10 closures.
The group is paying an interim dividend of 4.3 pence, up 59.2% and a special dividend of 25 pence, worth 250 million pounds.
(Reporting by James Davey,; Editing by Sarah Young and Keith Weir)
By James Davey