The slide is expected to continue, perhaps right up until Oct. 31, the day Britain is scheduled to depart the European bloc. The moves are now gradual, reflecting a continued downgrading of sentiment rather than a meaningful change to the economy. But as Oct. 31 draws closer, the pound could plunge. Britain could well descend into recession.

Details: The currency has lost nearly 3 percent of its value against both the American dollar — it fell to $1.2151 on Tuesday afternoon — and the euro since Mr. Johnson took over last week.

Big picture: The decline is at once a reflection of the market’s recognition that Britain has been economically weakened by the looming Brexit, and a cause for distress. Most broadly, it signals that investors see less need for British currency in the future, because Brexit is already reducing the appeal of doing business in Britain .