British holidaymakers travelling to popular European destinations have been warned to be vigilant after a militant organisation protesting against mass tourism promised fresh attacks.

The leader of a group of Spanish anarchists accused the travel industry yesterday of “destroying” Barcelona and the Balearic islands and vowed direct action. Travel operators said that the protests could escalate and pose a risk to holidaymakers.

This week British tourists on a sightseeing trip in Barcelona thought that they had been ambushed by terrorists when masked men attacked their open-top bus. The vehicle’s tyres were slashed and slogans daubed on its windows.

The same group then burst the tyres of bicycles rented by tourists and smashed windows in five-star hotels. On the same day activists in Palma, Majorca, let