Vodafone is reportedly close to securing a deal that would make it the third largest telecoms operator in Japan.
It is believed that Vodafone is close to buying a further 10% of Japan Telecom.
If the deal to buy AT&T's stake in Japan Telecom goes ahead, it would give Vodafone the edge over BT in the world's second largest mobile communications market.
The deal would leave Vodafone with a 25% stake in Japan Telecom, compared with BT's 20%.
Observers say BT's weak financial position has left it with little chance of competing for the AT&T holding.
The future of AT&T's stake in Japan Telecom has been in question since last autumn, when Japanese wireless company NTT DoCoMo agreed to buy 15% of AT&T Wireless.