On Monday, most travel money firms were offering $1.17 for every pound, the lowest rate since 1986.
The slide in sterling means holidays overseas will be more expensive for UK travellers.
Travelex's tourist rate is down 19% from its strongest level against the dollar over the past year.
It stood at just under $1.44 shortly before the UK's European Union referendum in June.
The fall against the euro has been less dramatic. The pound now stands at a little over 1.10 euros, the lowest since 9 November.
Sterling dropped sharply in Asian trade on Monday, before recovering some ground, following reports that Theresa May will signal that the UK is planning a "hard" exit from the European Union when she gives a speech on Tuesday.
UK travellers buying US dollars for holiday money are getting the lowest rates for nearly 31 years, according to the exchange firm, Travelex.