LONDON—The British government has relinquished control of the former Royal Bank of Scotland for the first time since it bailed out the lender during the global financial crisis almost 15 years ago. The Treasury said Monday that it had sold 1.2 billion pounds ($1.6 billion) of shares back to the bank, now known as NatWest Group, cutting the government’s stake to 48.1 percent, from 50.6 percent. Royal Bank of Scotland came to the brink of collapse in 2008 after a global acquisition spree that briefly made it the world’s biggest bank but also left it heavily exposed to risky loans in the United States. The government owned as much as 84.4 percent of the bank after investing 45.5 billion pounds in 2008 and 2009. The stake has been gradually reduced through a series of share sales that began in 2015. “This sale means that the government is no longer the …