The trading dilemma is reflected in the decline in the number of speculative bets placed on whether the pound will strengthen or weaken. The combined number of long and short sterling positions fell to 75,858 in the seven-day period to March 26, the latest Commodity Futures Trading Commission data shows. That was the smallest number since 2012.

The most marked drop has been in short positions, with bearish investors throwing in the towel in the past couple of weeks as parliament sought an alternative to Prime Minister Theresa May's thrice-rejected deal. But such shifts in positioning have failed to eject sterling from the range in which it has been trading for more than a month, between roughly $1.2950 and $1.3380. That may be why Bank of France boss Francois Villeroy de Galhau said on Tuesday that financial markets, including the foreign exchanges, needed to price in the growing risk of a no-deal Brexit.

Yet investors cannot use the spot currency market to do this without losing money if Britain chooses a less damaging route out of the EU or not to leave at all. The scale of their concern is, however, writ large in the options market.

Implied volatility, which reflects how much exchange rates are expected to gyrate, is at least twice as high for sterling/dollar than it is for euro/dollar for options that expire in six months' time or less. Even more notable is that implied volatilities are more elevated for shorter-dated sterling/dollar options than for longer-dated ones - unusual, since traders normally expect volatility to be higher the further into the future they look.

All of which makes sense. Traders may not be brave enough to place big bets on where sterling is heading while the outlook is so murky. But the data implies huge moves in the currency markets at the first sign of clarity.