The pound was down 0.9 per cent against the US dollar at $1.2961 at lunchtime, below the crucial $1.30 mark which the currency has settled around in recent weeks. Sterling also set a new five-week low against the euro as it sunk one per cent to €1.1554.
"The pound is back under pressure this morning," said Chris Saint, senior analyst at Hargreaves Lansdown.
"Markets look to have at least one eye on Brexit's timing after Boris Johnson stepped into the fray to suggest the formal withdrawal process could begin early next year."
Alexandra Russell-Oliver, an analyst at Caxton FX agreed: "Comments from foreign secretary Boris Johnson that the UK might trigger Article 50 early next year and be out of the EU within two years may have weighed on the pound, which reversed some of its earlier gains in overnight trading."
The pound's move on rumours about the timing of Article 50 supports the view of most analysts that the main driver of the currency over the coming years will be speculation about the shape of the UK's exit deal.
This isn't the first time the former mayor of London has been blamed for swings in the value of the pound. As he dramatically announced he would not be standing to succeed David Cameron as prime minister, the pound rose 0.6 per cent against the dollar.
Correlation, or causation?