Statement from Assistant Commissioner Mark Rowley, the UK lead for Counter Terrorism Policing:
"Our thoughts are with Spain, the victims, their families and all the other people affected by yesterday's barbaric attack in Barcelona.
"We have offered our support and assistance to the Spanish authorities who are continuing to respond to the terrorist incident in the busy tourist area of Las Ramblas, which killed a number of people and injured many others. This was followed by a second incident in the seaside resort of Cambrils, south-west of Barcelona.
"We are liaising closely with the Spanish authorities and await further updates regarding the nationalities of the deceased and injured.
"Officers are at UK ports today ready to meet British holiday-makers returning to the UK from the region in order to take any witness accounts. Anyone who has returned from Spain can also call the confidential hotline with information on 0800 789 321. We are also urging people who have images or footage that may assist investigators to upload them to Ukpoliceimageappeal.com
"The current threat is global and it is only by working together that it can be defeated. Our network of counter-terrorism police liaison officers posted in locations around the world help us connect investigations internationally. We have a long tradition of collaboration with the very capable Spanish police on terrorism matters.
"In the UK the current threat from international terrorism remains 'severe' meaning an attack is highly likely. Activity by police and the security services continues around the clock to disrupt and prevent further attacks in the UK.
"We remain on a heightened state of readiness and are continually reviewing our security arrangements to reflect the threat we are facing. We ask that people remain alert, vigilant but not alarmed and report anything suspicious to us at the earliest possible opportunity.
"People with information about those suspected of being involved in terrorism should call the confidential hotline on 0800 789 321 or gov.uk/ACT. In an emergency dial 999.
"Earlier this summer, UK police launched a video for holidaymakers travelling abroad providing safety advice in the rare event that they are involved in a terrorist attack.
"The key advice is to run to a place of safety. If there is nowhere to run then hide. It is better to hide than confront. Remember to turn your phone to silent and turn off vibrate. Barricade yourself in if you can. Then finally, and only when it is safe to do so, tell the authorities by calling the emergency number which is 112 for the EU. For other information check the following site: gov.uk/travelaware
"The advice to anyone caught up in a terrorist attack in the UK is the same - Run, Hide, Tell by calling police on 999."
Note to editors: Video with advice for holidaymakers: https://youtu.be/kzcldaLbYPA