China has abandoned a solemn pledge to keep its exchange rate stable and is carrying out a systematic devaluation of the yuan, sending a powerful deflationary impulse through a global economy already caught in a 1930s trap.

The country's currency basket has been sliding at an annual pace of 12pc since the start of the year. This has picked up sharply since the Brexit vote, suggesting that the People's Bank (PBOC) may be taking advantage of the distraction to push through a sharper devaluation.

"This makes a mockery of the PBOC's suggestion that its policy is to keep the currency's value stable," said Mark Williams, chief China economist at Capital Economics. "Markets will not take PBOC policy statements at face value in the future."