Lawyer Ben Nicholson lost his wife Anita, son Alex, 14, and daughter Annabel, 11, when one of seven suicide bombers struck as they ate breakfast at the Shangri-La hotel in Colombo.
Londoner Matthew Linsey's daughter Amelie, 15, and son Daniel, 19, were killed in the same blast on the final day of their holiday.
GP Sally Bradley and her husband Bill Harrop, a retired firefighter, from Manchester, died in the Cinnamon Grand Hotel bombing.
Eight Britons were among at least 310 people killed in the explosions, while more than 500 people were wounded. Police have arrested 40 suspects as of Tuesday morning.
No group has claimed the attacks, but Sri Lankan officials have named little-known Islamic extremist organisation National Thowfeek Jamaath.
The seven suicide bombers were all Sri Lankan citizens but the group is believed to have links with foreign terrorist networks.
Mr Nicholson said his family had been visiting Sri Lanka for a holiday from their home in Singapore. Mrs Nicholson, who was also a lawyer, worked for mining and metals company Anglo American, while Mr Nicholson is a partner with law firm Kennedys.
"Mercifully, all three of them died instantly and with no pain or suffering," he said in a statement on Monday.
"I am deeply distressed at the loss of my wife and children. Anita was a wonderful, perfect wife and a brilliant, loving and inspirational mother to our two wonderful children.
"The holiday we had just enjoyed was a testament to Anita's enjoyment of travel and providing a rich and colourful life for our family, and especially our children.
"Alex and Annabel were the most amazing, intelligent, talented and thoughtful children and Anita and I were immensely proud of them both and looking forward to seeing them develop into adulthood."
Local media reported Mr Nicholson desperately searching for his family after the blast.
Mr Harrop and Dr Bradley had been living in the Australian city of Perth since 2013 where Dr Bradley was practising medicine, but were due to return to the UK soon, where they had bought a retirement home in the Cotswolds.
Dr Bradley's brother, former Labour MP Lord Keith Bradley, said: "She was truly a bright light in many people's lives.
"The light may have been cruelly distinguished for no reason or justification, but she will always live in our hearts and the memories she provided will be forever cherished. I, and my family, will miss her more than words can articulate."
Dr Bradley had worked as director of clinical services at Rockingham Peel Group in Perth.
The company's executive director Kathleen Smith said Dr Bradley and her husband "lived for each other".
Ms Smith told Perth-based radio station 6PR Radio: "She absolutely loved living in Australia. She felt very at home here.
"They were soulmates, they just lived for each other."
Mr Harrop had two sons from a previous relationship, Miles and Gavin. Gavin had been holidaying with the couple at the time of the blast but was staying at a different hotel.
"He had two boys, which Sally took on as her step-sons. She talked about them as if they were her own," Ms Smith said.
Ms Smith added Dr Bradley told her they were staying at a "posh" hotel for a few days, then would move to other parts of Sri Lanka.
She said the sociable and adventurous couple did not usually stay at five-star hotels.
"She would normally get out amongst the community that she was visiting," Ms Smith said.
Mr Harrop retired from Greater Manchester Fire and Rescue Service in 2012 after 30 years as a firefighter and was decorated for his role in the aftermath of the 1996 IRA attack on Manchester.
Assistant County Fire Officer Dave Keelan paid tribute to his former colleague.
He said: "This is devastating news and the thoughts of Greater Manchester Fire and Rescue Service are with Bill and Sally's families.
"Bill served here for 30 years, retiring at the end of 2012. He was a much-loved and respected colleague and friend. He will be greatly missed."
Mr Linsey, an investor in emerging markets, told The Times his children were born in Britain but had dual US-UK citizenship because he was born in the US.
He said: "Amelie was really fun. She was smart, beautiful. Very loving, very caring, understanding. She cared about her family and her friends. And the same with Danny."
It emerged on Monday morning that Danish billionaire Anders Holch Povlsen lost three of his four children in the attacks.
Mr Holch Povlsen is the largest stakeholder in online fashion retailer Asos and is believed to be the largest private landowner in Scotland after buying a string of estates.
Hampstead and Kilburn MP Tulip Siddiq said she had lost a relative in the attacks, while Foreign Secretary Jeremy Hunt said the UK would offer Sri Lanka support in the days to come.
He said the terrorist attacks were "absolutely devastating and despicable" and "for this to happen on Easter Sunday is something that will shake people around the world, of all faiths and none, to the core".
Union flags on Downing Street and the Foreign Office building are due to be flown at half mast on Tuesday in mourning for the victims of the attack, the FCO said on Monday evening.
One line of the Sri Lankan inquiry will be what intelligence services knew about the attack, with telecommunications minister Harin Fernando tweeting: "Some intelligence officers were aware of this incidence.
"Therefore there was a delay in action. What my father heard was also from an intelligence officer. Serious action need to be taken as to why this warning was ignored."
A curfew was imposed on Sunday night and social media use was also restricted by authorities, which claimed the move was to prevent the spread of false information.
Sri Lanka's prime minister Ranil Wickremesinghe condemned "the cowardly attacks on our people".
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.