LISBON (Reuters) - Haunted by the idea that Brexit may affect Portugal’s booming tourism industry, a government agency has launched a campaign to say “Brelcome” to the thousands of British visitors who choose the country as a holiday destination.

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“Portugal will never leave you” is the main slogan of the “Brelcome” campaign, which has already reached around 6 million Britons through social media and other digital platforms, the Turismo de Portugal agency said.

Holiday seekers from Britain are Portugal’s largest tourism market, representing 20 percent of the industry, but last year they already stayed fewer nights, dropping 7.3 percent compared with 2017, according to official data.

The pound tanked after the June 2016 referendum result and has since gyrated wildly on any Brexit news - potentially making destinations outside the euro zone, such as Turkey, more attractive.

If Britain ends up crashing out of the bloc without a deal, the pound could drop about 5 percent, a Reuters poll showed. The European Union on Wednesday gave Britain six more months to leave.

Spain, the most popular destination for British sunseekers who accounted for 9 percent of foreign visitors last year, is seen as the biggest loser from a no-deal Brexit, which could wipe off more than $1 billion (764.7 million pounds) in spending between 2019-2025, according to Euromonitor.

Taking pre-emptive action, Portugal’s campaign, launched last month, features a series of videos saying its beaches, golf courses and wine cellars will always be there, with or without Brexit.

“We thought this was the right opportunity to tell a very good friend that we will never leave them,” president of Turismo de Portugal Luis Araujo told Reuters, noting that the two countries are the world’s oldest allies, forged through a 1386 treaty.

“We have been growing in every market for the past three years, particularly in the United States and Brazil, but we will never have a market as big as the UK so it’s really important for us.”

Portugal has said British tourists will not need a visa even in the event of a no-deal Brexit and said airports in the Algarve and Madeira would open separate customs lanes for British visitors to ease entry.

No-deal legislation to protect the rights of British citizens living in Portugal was also approved by the government last month.