The Foreign Office has updated its information for British citizens in Sri Lanka or for those planning to visit the country.
A number of bomb explosions have killed at least 190 people and Britons are thought to be among those who have lost their lives.
The bombings at churches and luxury hotels have been widely condemned.
Prime Minister Theresa May said the Easter Sunday massacre was "truly appalling" and "no one should ever have to practise their faith in fear".
Sri Lanka explosions: Brits believed to be among those killed in eight blasts in churches and hotels
On their website the Foreign Office says: "On 21 April 2019 bombs were used to attack three churches and three hotels in Sri Lanka, in central Colombo, and in the northern suburb of Colombo Kochchikade, and roughly twenty miles north of Colombo in Negombo and in the east of the country in Batticaloa.
"There have been significant casualties. Security has been stepped up across the island.
"The airport is operating, but with increased security checks.
"Some airlines are advising their passengers to arrive early for check-in, in light of increased security screening.
"If you are in Sri Lanka, please follow the advice of local security authorities, hotel security staff or your tour company. You should avoid large gatherings.
"If you are in Sri Lanka and you are safe, we advise that you contact family and friends to let them know that you are safe.
"If you are in Sri Lanka and have been directly affected by the attacks, please call the main number for the Embassy in Colombo: +94 11 5390639, follow the phone message, and select the emergency option from where you will be connected to one of our consular staff."
The UK's High Commissioner to Sri Lanka James Dauris was in Colombo with his family at a church service which was cut short by the attacks, The Mirror reports .
He tweeted: "Our prayers for the victims of these evil attacks, and for their families. Our thoughts are with the medical staff, police and all involved in the response."
In Colombo, St Anthony's Shrine and the Cinnamon Grand, Shangri-La and Kingsbury hotels were targeted.
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.
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."
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