LONDON — The pound tumbled against the dollar on Tuesday, as investors grew worried about Britain’s economic prospects and the specter of a no-deal Brexit.

The pound has been sliding during the last few weeks, but on Tuesday, it slipped as much as 0.5 percent, falling to $1.24, its lowest level in two years. The fall below $1.25 was significant, said Lee Hardman, a currency analyst at MUFG Bank, suggesting to investors that there were broad worries about the British currency. “It opens the door to further downside potential,” he said.

“There are very few positives for the pound,” he added.

Among the concerns was the growing likelihood that Boris Johnson, the candidate for Conservative Party leader who has talked about the need to leave the European Union by the Oct. 31 deadline with or without an agreement, would become the next prime minister. Business leaders have warned against a no-deal Brexit, and the prospect of a victory by Mr. Johnson was “making market participants more nervous,” Mr. Hardman said.

Economic growth has been sluggish in Britain this year. Gross domestic product rose 0.3 percent in the first quarter, and on Tuesday, Dean Turner, an analyst with UBS Global Wealth Management, said the economy probably contracted in the second quarter.