At least 138 people have been killed and more than 400 injured following multiple explosions at hotels and churches.
In a video message James Dauris, said: "We understand that some British citizens were caught in the blasts but we are unable to say how many people are, or might have been affected."
Dauris said he was on his way to a hospital in Colombo where victims had been taken.
Message from British High Commissioner on incidents in #SriLanka.— UKinSriLanka\uD83C\uDDEC\uD83C\uDDE7\uD83C\uDDF1\uD83C\uDDF0 (@UKinSriLanka) April 21, 2019
If you are a British National in Sri Lanka follow the advice of the local authorities and travel advice, updated here https://t.co/cxgzYCPU49
Our thoughts are with the innocent people who have suffered today pic.twitter.com/txgsv2I3yZ
The Shangri-La, Cinnamon Grand and Kingsbury hotels in the capital Colombo were among those hit, while three churches in Negombo, Batticaloa and Colombo’s Kochchikade district were targeted during Easter services.
Authorities believe at least eight blasts to have taken place.
The Foreign Office has advised any Britons "directly affected by the attacks" to call the British embassy in Colombo on +94 11 5390639, follow the phone message, and select the emergency option from where they will be connected to consular staff.
"If you’re in the UK and worried about British friends or family in Sri Lanka caught up in the incidents, please call the FCO switchboard number: 020 7008 1500 and follow the same steps," the FCO added.
A curfew has been imposed from 18:00 to 06:00 local time (12:30-00:30 GMT) and a temporary block on major social media networks imposed by the Sri Lankan government.
No group has yet claimed responsibility.
Long-haul specialist operators have reacted to the news in the UK with help and advice for trade partners and messages of support to the Sri Lankan people.
Brad Bennetts, Kuoni’s national sales manager, said the operator was “monitoring and assessing the situation”.
Posting on industry Facebook group Travel Gossip, he said: “Our operations team are getting in touch with every customer in resort at present.
“If any agents have any concerns regarding their bookings they are welcome to contact the Kuoni emergency number or email me on firstname.lastname@example.org or 07917058014."
Derek Jones, chief executive of Kuoni parent, Der Touristik UK, added on Twitter: "Thoughts are with the people of Sri Lanka including many good friends and with everyone affected by today’s awful events. Such a pointless waste of so many innocent lives.”
Gold Medal said "following the tragic incidents" in the destination, "we are contacting all customers who are currently in resort to offer support and assistance. Should agents or customers need to contact us please call our out-of-hours team on 01772 550185".
Earlier this morning, Andy Freeth, If Only chief executive, described news of the bombings as “horrific” and confirmed that all of its ground handling team and customers were safe.
"Our thoughts are with the families affected by this awful tragedy,” he tweeted.