Artist Benjamin Sullivan has said winning the BP Portrait Award after being shortlisted 13 times, is a “wonderful surprise and accolade”.
Sullivan won with Breech!, a tender picture of his wife Virginia breastfeeding their daughter Edith.
The judges said it reminded them of Madonna and Child portraits and showed “the depth of the maternal bond”.
“For a portraitist, the BP award is the only gig in town – it’s the most important thing,” he told BBC News.
Sullivan, who was born in Grimsby and lives in Suffolk, receives £30,000 and a gallery commission worth £5,000 at the National Portrait Gallery, London.
He said he wanted to show the “bond and connection” between mother and daughter with Breech!.
It is now going on show to the public as part of the gallery’s annual exhibition of the shortlisted works, and Sullivan said his award meant even more to him knowing so many people would get to see his art.
“Hundreds of thousands of people see the show so it’s wonderful from that point of view. From every angle it’s great news.”
He came third in last year’s competition, for a portrait of poet Hugo Williams and has overall been selected a record 13 times.
Broadcaster and judge Kirsty Wark said of Breech!: “The woman is tired. She is in love. Her life has changed forever. We know her.”
The 40-year-old artist often paints family members, saying that “the emotional connection between sitter and artist is at the root of all successful portraits”.
“I’ve done lots of paintings of Ginny, my wife, throughout our 10-year relationship so it felt normal to do this now in this situation, with our first child,” he told BBC News.
“It was great to see that bond and connection between them so it was important to do from that point of view.”
Sullivan said he wanted the portrait to celebrate the new arrival, as well as mark the difficult time he and his wife had during the birth of Edith – who was a breech baby, as referred to in the title of the portrait.
He added that it was a time when “a sense of calm descended after the usual period of disarrangement that new parents face”.
Sullivan told BBC News: “I just wanted to do a painting of the two of them… the child gets a lot of comfort from the physical thing of breast-feeding.
“Talk to any midwife and they will tell you that getting young mothers to breast feed and feel confident in public and normalise it is really important.”
He said he had seen “every kind of reaction to the work”, including negative, but those “in the know” – midwives and mothers with young babies – say, “Oh my God, you’ve got it just so”.
Breech! was painted over a four-to-five-week period in his studio “when Edith’s co-operation was forthcoming”.
Thomas Ehretsmann was awarded second prize, and £10,000, for Double Portrait – showing his wife Caroline walking in a park when she was eight months pregnant.
The judges said the portrait, painted with layers of semi-transparent acrylic paint, showed Caroline’s “sense of stillness, strength and serenity”.
The third prize of £8,000 went to Antony Williams for his portrait of friend Emma Bruce, simply titled Emma, and the judges said his “distinctive technique” made the painting look “almost sculptural”.
The model has been sitting for Williams for 11 years at his studio in Chertsey, and he said he wanted to show her vulnerability and determination.
New Zealand artist Henry Christian-Slane won the BP Young Artist Award of £7,000 for Gabi, showing his partner Gabi Lardies. The 26-year-old said he believes the painting – being of someone he knows so well – “balances being analytical and instinctual”.
The BP Portrait Award 2017 exhibition is at the National Portrait Gallery from 22 June to 24 September and admission is free.