More than four out of five tourism firms believe staying in the EU is important to their businesses, according to new research.
Trade association UKinbound found that 82% of its 300-plus members want the UK to remain in the EU following the upcoming referendum.
The organisation's chairman, Mark McVay, warned that Britain could be isolated if a so-called Brexit takes place.
Addressing the UKinbound annual convention in the Scottish resort of Aviemore, Mr McVay said: "The Prime Minister recently asked the business community not to be shy about stating their views on EU membership. Well Mr Cameron, let the inbound industry be the first to say 'yes' to remaining in Europe."
He described how Britain's tourism industry "thrives on collaboration with our European neighbours" and insisted that it made financial sense to stay in the EU.
"We want to make it easier for people to travel to our beautiful country, not isolate ourselves from a market which collectively brought two-thirds of our business in 2015," he explained.
"While the EU is not perfect and could be improved, staying part of it makes sound economic and - from our perspective - commercial sense."
Bernard Donoghue, chairman of Tourism Alliance, which represents the industry, said his personal point of view was that it would be "an absolute tragedy to leave the EU".
He predicted that Brexit would lead to a perception that Britain is "consolidating its own image as a nation of little Englanders".
Mr Donoghue also warned that politicians who use "inflammatory rhetoric" about immigration are sending a message to potential visitors that they are not welcome.
He spoke about a number of high-profile figures who have enjoyed successful careers in the UK and were born abroad, such as television presenter Loyd Grossman and celebrity chefs Antonio Carluccio and Michel Roux.
Mr Donoghue said: "Politicians - stop the inflammatory rhetoric about immigration. Stop it.
"Because the message that sends out to the world is that we're not welcoming and we prefer closed doors.
"You can bet your life when politicians talk about 'we don't want economic migrants or we don't want immigrants here', you know they don't mean Loyd Grossman or Antonio Carluccio or Michel Roux, or other people who have been born outside of the UK, like (London mayor) Boris Johnson.
"You know they don't mean those people. So let's call them on it."
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