There is growing speculation that Royal Bank of Scotland may dump its Bank of China stake as a three-year lock-up on the shares expires today.

Bank of China fell 2.3% in Hong Kong to HK$2.14 (19p), extending this year's loss to 43%. BoC says RBS has not said it intends to sell its stake.

"If RBS plans to sell our shares, they need to inform us in advance. So far, we haven't received such notice," Bank of China's investment relations official Luo Nan said.

RBS invested in BoC in 2005, before BoC went public. RBS and three foreign strategic investors, including Singapore's Temasek and Swiss bank UBS, paid a combined $6.78 billion (£4.69 billion) for a 21.85% stake.

RBS's 4.3% stake in BoC is worth more than £1.5 billion.

Sir Fred Goodwin, the now-discredited mastermind of RBS expansion including the BoC stake, is still on the BoC board and will remain there until 31 January.

RBS is reviewing its sprawling empire after a UK government rescue and will decide in the next two months whether to go ahead with a £7 billion sale of its insurance unit. That disposal of Churchill and Direct Line seems to have stalled, with RBS rejecting an offer from private-equity group CVC Capital Partners.

Reports suggest Stephen Hester, the new chief executive, is keen to retain Churchill and Direct Line. He wants to refocus RBS on its roots in the UK, which has only fuelled speculation of a reduction of its BoC holdings.