Around 2.3 million Brits travelled to Europe by ferry and aeroplane without insurance in the past 12 months because they believed they were already covered.
Research by American Express Insurance Services showed that these people incorrectly thought the European Health Insurance Card (EHIC) gave them sufficient cover to travel within the EU.
However, EHICs only provide reduced or free healthcare to Brits and do not cover holiday cancellation, curtailment or repatriation, the firm explained.
Chris Rolland, head of the company, warned that with so many people from the UK intending to visit a European country such as Germany or France this month, they must ensure they have adequate cover.
"[EHICs] are not a substitute for comprehensive travel insurance so you will not be covered for delays, loss of baggage or cancellations or experience other benefits of having an independent travel insurance policy," he explained.
Research by the Association of British Travel Agents found that nine per cent of Brits are considering travelling by ferry and taking their family on a driving holiday to Europe this year.