The philanthropists who sent Britain's "orphans" thousands of miles overseas to farms in Australia and Canada believed they were performing a charitable deed.

Between the 1920s and the 1960s as many as 150,000 young children were despatched to institutions and foster homes abroad so that they might begin happier lives in the under-populated Commonwealth.

Charities including Barnardo's, the Catholic church and local authorities helped organise the emigration of youngsters aged between three and 14. So the children could make a clean start, they were usually told their parents had died.

In reality, many were children of single mothers who had been forced to give them up for adoption in an era when their solitary status constituted a grave social stigma.

The fresh beginning the children were promised degenerated into years of servitude and hard labour on remote farms and at state orphanages. They were often subjected to physical and sexual abuse, separated from their siblings and taunted for being "the sons of whores".

The official Child Migrants Programme, which ended 40 years ago, ruined the lives of the most vulnerable. It has taken decades for the harm and emotional damage to be acknowledged.

Gordon Brown's apology, coming several months after the Australian prime minister, Kevin Rudd, performed a similar act of public atonement, is intended to help the process of healing for survivors.

Last November Rudd, speaking to a gathering of 1,000 victims known as the "Forgotten Australians" in Canberra, declared: "We are sorry. Sorry that as children you were taken from your families and placed in institutions where so often you were abused.

"Sorry for the physical suffering, the emotional starvation and the cold absence of love, of tenderness, of care. Sorry for the tragedy – the absolute tragedy – of childhoods lost."

It was "an ugly story" and a "great evil" had been done, Rudd admitted. He hoped the national apology would become "a turning point in our nation's story".

Marcelle O'Brien was four when she was sent to Australia. Now aged 65, she was born in Worthing, west Sussex, and had lived with a loving foster family since she was 13 months old.

Nonetheless she was transported to Pinjarra, western Australia, where she was placed at Fairbridge farm school, about 50 miles from Perth. "I was neglected and abused in a harsh institution when I had the option of love and a family life with my foster mother in England," O'Brien said. "I lost my whole childhood and with it my sense of hope and joy."

At Fairbridge, life was cruel and brutal, she said. Children had no shoes or coats and were physically, mentally and sexually abused. "The best way to warm our feet was to tread in a fresh cow pat." Girls carried out domestic chores while boys did farm work.

They were not allowed to mix and siblings were banned from speaking, she said. When she turned 16, O'Brien was forced into domestic service looking after a baby and was very lonely.

O'Brien, now a great-grandmother, was finally reunited with her mother Kitty eight years ago with the help of the Child Migrants Trust (CMT). "I just held her in my arms, and perhaps a little of the hurt began to melt away. My mother was then quite frail and I didn't have her for very long, but now I have an identity and that can never be taken from me again."

Another of those sent overseas was Tony Costa, 68, from Islington, London. He is still haunted by experiences at Bindoon Boys Town, a Christian Brothers institution near Perth. "I still wake during the night in a cold sweat, in a state of night terror featuring the monsters of my childhood – though it was never any kind of childhood," he said.

"I vividly recall crying myself to sleep, pleading with God to save me from the torment of my life every day. Desperately trying to understand what crime I had committed to warrant such a heinous punishment as to be incarcerated at Bindoon."

His mother had given him to nuns to look after. He grew up believing he was an orphan. By the time the CMT was able to retrieve his papers from the church, his mother had died.

Costa, who went on become mayor of the western Australian town of Subiaco, said: "At Bindoon ... there were endless tasks: mixing cement, making bricks, carting them up the ramp without any safety measures, no shoes so whenever you dropped a brick it would land on your bare feet.

"It was severe child labour, exploitation of the worst kind, all being done to 'the glory of God' which was little comfort for starved and beaten young boys. The brothers acted like overseers on a chain gang, shouting and whipping us if we fell behind."

Ian Thwaites from the Child Migrant Trust said it was "still very difficult to accept the full extent of what happened". The issue of compensation was up to the child migrants themselves to consider, he added.

The chief executive of Barnardo's, Martin Narey, told BBC radio: "If individuals in Australia think we can help and do anything to put right the hurt or distress that has been caused, I urge them to contact me personally."