Fear that a no-deal Brexit could cause the pound to fall to parity against the dollar for the first time in its history have been renewed this week. The prospect rose when, in a television debate, both candidates in the Conservative leadership race – Boris Johnson and Jeremy Hunt – refused to rule out the scenario.

In response, the pound has fallen to a two-year low of $1.24 and is 17pc lower than before the referendum in June 2016.

Many have suggested the British currency could tumble as low as $1.00-$1.10 if the country crashes out of the EU at the end of October.

It would represent a further 19pc fall for the currency, rivaling the 25pc drop on Black Wednesday in 1992, when the UK was forced to withdraw from the European exchange-rate mechanism.

But the recent movements are only the tail end of a more than 200-year relationship between the two currencies.

For most of the 1800s until the start of the First World War, every £1 was worth just under $5. The Napoleonic wars, which weakened the pound, was one exceptional period; as was the US Civil War, which saw the pound temporarily spiking up to $10.

Over much of this period the "gold standard" was in force, which required nations to back the value of their banknotes with the equivalent in gold, establishing stability in exchange rates.