LONDON, Dec 7 (Reuters) - Britain's spending watchdog may open an investigation into the government's decision to start selling shares in Royal Bank of Scotland last year causing a 1.1 billion pound ($1.39 billion) loss for taxpayers, sparking accusations of poor timing from opposition politicians.

The head of the National Audit Office, said in a letter to the Treasury Select Committee, he is considering a review of the government's handling of the taxpayer's investment in the nationalised banks, including the advice given by UK Financial Investments, the body that manages the stake in the bank, to start selling the shares at a loss.

"I am still finalising the exact scope and nature of my work in this area," Amyas Morse, head of the National Audit Office, said in a letter to the head of the Treasury Select Committee.

"It is also likely that this will extend to a number of case studies, including the decision to sell RBS shares in August 2015."

Politicians have previously questioned whether the advice given by the UK Financial Investments was listened to and whether restarting the sale when the shares were below the amount the government paid for them represented the best value.

The British government said last month it faces an almost 27 billion pound loss from rescuing failed banks during the 2007-2009 financial crisis after a slump in the lenders' value since Britain's vote to leave the European Union in June.

Taxpayers currently face a 32.4 billion pound loss on the value of the government's shares in Royal Bank of Scotland, according to the Office for Budget Responsibility, Britain's independent budget watchdog.

UK Financial Investments declined to comment. ($1 = 0.7935 pounds)

(Reporting By Andrew MacAskill, editing by Anjuli Davies) ((Andrew.MacAskill@thomsonreuters.com; +442075421726; Reuters Messaging: andrew.macaskill@thomsonreuters.com))