Primark's flagship Oxford Street store has generated sales of nearly £200m since it opened in April last year.
The value fashion giant's flagship store, which is about 70,000 square foot, is thought to be delivering average weekly sales of between £2.5m and £3m.
The revelation comes ahead of a third-quarter trading update for Primark's owner Associated British Foods tomorrow, which is expected to show that Primark's sales growth has slowed but remains ahead of its high-street rivals Marks & Spencer and Next.
The sales at its flagship store illustrate the extent to which Primark is grabbing sales from rivals such as M&S, which has a huge store opposite on Oxford Street.
When Primark opened its Oxford Street store in April 2007, shoppers fought and scrambled over prostrate bodies to be the first inside after false rumours of a sale.
The Panmure Gordon analyst Graham Jones forecasts that Primark's like-for-like sales growth will be flat for its third quarter and second half, which it has scaled back from a previous forecast of a slowdown to 2 per cent for the second half.
Mr Jones said: "The deterioration in trading conditions on the UK high street is plain for all to see, and with M&S and Next, on our estimates, seeing clothing like-for-like sales declines of 7 to 8 per cent recently, it would be unrealistic to think Primark will not be affected."
However, Mr Jones said: "We believe that Primark has been resilient in recent months, with April actually the most difficult period, due to the much colder weather than April 2007."
Primark is also strengthening its senior management team. It has poached Paul Marchant, New Look's chief operating officer, for the same role at Primark. The hiring of Mr Marchant sparked speculation that Primark's founder and executive chairman, Arthur Ryan, who will be 73 this month, is preparing to take a back seat.
Primark is also hiring Guy Young, a C&A executive on the Continent, as international trading director, as part of the retailer's plans to open stores in Holland, Portugal and Germany next year. Mr Young will head the buying team for Ireland and Europe.
Primark has 177 stores in Ireland, where it trades under the Penneys brand, Spain and the UK.