Carphone Warehouse is to accelerate its joint venture with Best Buy, the American electrical retail giant that rumoured to be a possible takeover suitor for Charles Dunstone's group.

Carphone, which begins selling the much-vaunted iPhone tomorrow, said that it would roll out 1,000 stores in the US in the next two years.

Best Buy recently bought a 3 per cent stake in Carphone, with some analysts suggesting that it was the first stage in a takeover campaign.

About 70 stores have been piloted in the US, both as stand-alone Best Buy Mobile shops and mobile units within general Best Buy stores.

The scale of the project has been increased drastically.

Before this morning's announcement up to 200 stores were expected.

Mr Dunstone, the chief executive who founded Carphone Warehouse with £6,000 in 1989, said: "We have made good progress across the group in the first half. The retail business continues to prosper across Europe, and we are announcing today a major roll-out of our US venture."

Carphone said: "Our US trial has continued to go well, giving us the confidence to pursue a roll-out to 1,000 Best Buy stores over the next two years.

"In addition, we are exploring opportunities to expand the venture into other markets where Best Buy has a presence."

Canda has been earmarked as a possible area of expansion.

The American partnership will entail the Carphone Warehouse business model of a one-stop-shop that offers customers the choice of every operator, brought under the Best Buy brand.

Accompanying the US news was a rise in pre-tax profit of £14 million last year to £56 million for the first half of the year to September 29, in line with expectations.

Revenue, at £2.14 billion, was slightly below analysts' forecasts.

Carphone said that it expected a strong second half, buoyed by the release of the iPhone, which it stocks exclusively.

But it gave warning that "off the page" orders for broadband subscriptions had been "much slower" in the first half of the year.

Carphone Warehouse shares fell 10p to 345.75p in early trading this morning, after strong gains this week.

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