People are getting less than half the sun protection they expect from suntan lotions, according to research.

But the problem isn't the lotion, it's our slapdash application of it, according to scientists at King's College London.

In theory, using sun protection factor (SPF) 15 sunscreen should be enough to stop sun damage.

But in reality, people need SPF 30 or 50 to be safe, say the study authors, who carried out experiments with volunteers in their lab.

Here consultant dermatologist, Dr Justine Kluk shows us how to apply sunscreen correctly and avoid sunburn.

Video Journalist: Laura Foster