LONDON—Bitcoin tumbled below the $40,000 mark on Wednesday to a three-and-a-half month low as selling in digital coins intensified after China banned financial and payment institutions from providing cryptocurrency services. Bitcoin, the biggest and best-known cryptocurrency, had already been under pressure from a series of tweets from Tesla boss Elon Musk, but the news from China sent it as low as $36,250, a 15 percent drop in the trading session. The cryptocurrency has tumbled 40 percent from a record high of $64,895 hit on April 14. It is also heading for its first monthly decline since November 2018. Bitcoin’s decline whacked other crypto assets, with Ether, the coin linked to the ethereum blockchain network, shedding as much as 28 percent on Wednesday to $2,426. It brings losses in the week since it hit a record high on May 12 to 40 percent. Meme-based dogecoin also tumbled—losing nearly 30 percent at …