At least five Britons are among more than 200 people killed in a series of bombings which ripped through churches and luxury hotels in Sri Lanka.
Foreign Secretary Jeremy Hunt said the death toll from the “horrifying” attack included “several British nationals”.
“To target those gathered for the simple act of worship on Easter Sunday is unspeakably wicked,” he said.
Prime Minister Theresa May said the Easter Sunday massacre was “truly appalling”, and “no one should ever have to practise their faith in fear”.
Three Britons and two holding joint US and British nationalities were killed, Sri Lankan authorities said, and the grim death toll could rise further.
Mr Hunt said the death toll of five Britons killed in the attack was “the latest figure that I have heard”.
“But obviously our High Commissioner is working on this with his team in the embassy in Colombo, working around the clock, and we are trying to gather as much information as we can about this,” he said.
“There is a crisis centre that has been set up and there is an emergency helpline on the Gov.UK website for anyone who has concerns about friends or relatives who may be holidaying in Sri Lanka.”
He said the terrorist attacks were “absolutely devastating and despicable” and “for this to happen on Easter Day is something that will shake people around the world, of all faiths and none, to the core”.
Mr Hunt said there was “no hard knowledge” about the perpetrators of the atrocity and “we obviously need to wait for the police in Sri Lanka to do their work”.
James Dauris, the UK’s High Commissioner in Sri Lanka, visited injured Britons in hospital in the capital Colombo and condemned the “deplorable violence”.
In Colombo, St Anthony’s Shrine and the Cinnamon Grand, Shangri-La and Kingsbury hotels were targeted in the first wave of explosions at shortly before 9am local time as worshippers attended morning services and tourists enjoyed their breakfasts.
Other blasts were reported at St Sebastian’s Church in Negombo, a majority Catholic town north of Colombo, and at Zion Church in the eastern town of Batticaloa.
After the near-simultaneous first set of attacks, there were further explosions in Dehiwala and Dematagoda on the outskirts of Colombo a few hours later.
The authorities said 207 were killed and 450 injured in the attacks, most of which were being blamed on suspected suicide bombers.
No one has taken responsibility for the killings, but officials say 13 suspects have been arrested and a safe house believed to have been used by the attackers has been located.
A curfew was imposed by the authorities on Sunday night.
The Easter attacks are the worst bloodshed Sri Lanka has seen since its brutal civil war ended a decade ago.
Mrs May said: “The acts of violence against churches and hotels in Sri Lanka are truly appalling, and my deepest sympathies go out to all of those affected at this tragic time.
“We must stand together to make sure that no one should ever have to practise their faith in fear.”
Britons in Sri Lanka who need help were urged to call the High Commission in Colombo on +94 11 5390639, while people in the UK worried about friends or family should call the Foreign Office on 020 7008 1500.
Labour leader Jeremy Corbyn called for “unity, love and respect” to combat hatred.
He said: “I’m appalled by the horrific attacks in Sri Lanka, on Easter Sunday, the most important day in the Christian calendar.”
Archbishop of Canterbury Justin Welby said: “On this holy day, let us stand with the people of Sri Lanka in prayer, condolence and solidarity as we reject all violence, all hatred and all division.”
Sri Lanka’s prime minister Ranil Wickremesinghe condemned “the cowardly attacks on our people”.
Nisanga Mayadunne, who studied at the University of London according to her Facebook profile, and her mother Shantha – a TV chef, are reported to be among the dead.
Nisanga posted a photo of her family eating breakfast in the Shangri-La on Easter Sunday.
Julian Emmanuel and his family, from Surrey, were staying at the Cinnamon Grand when the bomb went off.
He told the BBC: “We were in our room and heard a large explosion. It woke us up. There were ambulances, fire crews, police sirens.
“I came out of the room to see what’s happening, we were ushered downstairs.
“We were told there had been a bomb. Staff said some people were killed. One member of staff told me it was a suicide bomber.”