A Conservative minister has defended a decision to reduce benefits paid to widows and widowers with young children by claiming the old system risked stopping people from “readjusting” to life as a single parent.

Richard Harrington claimed that a societal shift in which women were more likely to work meant it was right to reduce the time over which payments were made. He also defended not extending the payments to unmarried but cohabiting couples by saying it might be too upsetting for them to provide evidence that they lived together.

Labour’s Stella Creasy accused the minister of using “justifications that sound like something from the dark ages” in a parliamentary debate over reforms to the widowed parent’s allowance – which is being renamed the bereavement support payment.

Government changes mean that from 6 April, bereaved parents will only receive payments for 18 months. Previously the payment lasted until children were 16 years old. The payment is based on the deceased spouse’s national insurance contribution and the maximum amount is £112.55 a week.

The benefit will also not be extended to include couples who are cohabiting but unmarried.

Creasy asked Harrington how the government could justify not paying the money for unmarried couples. Harrington claimed such a change would be expensive and complex to administer, adding: “Having to prove cohabitation could be a lengthy, complex process, which could cause distress at a time of bereavement.”

Pushed by the Labour MP on what had driven the decision to time-limit the benefit, he said 18 months was seen as the “most critical short-term time”.

“I hope the honourable lady will agree that the old system could be unfair and complex, and could act as a trap preventing people from readjusting,” he added, claiming that the allowances were first brought into place in 1925.

“The way that people thought in those days was that most women were wholly dependent on their husband’s income. If a woman was widowed, her sole source of income would disappear completely, so it was considered necessary to provide a replacement income for her to survive.”

He said women were now active in the workforce, with many households benefiting from dual incomes. “That is why we are modernising bereavement support into a simple, uniform and easy-to-understand benefit that better reflects society.”

A spokesperson for the Department for Work and Pensions said: “The old system – introduced more than 90 years ago – was based on the outdated assumption that a widow relied on their spouse for income, and would never work themselves. This doesn’t reflect people’s lives today.

“The new bereavement support payment focuses support on the 18-month period after the death of a spouse or civil partner, when it is needed the most.”

The spokesperson argued that the new payments would be easier to understand, wouldn’t be taxed and wouldn’t affect the amount received from other benefit payments, protecting those on low incomes.

“While couples do need to be married or in a civil partnership to be eligible for bereavement payments, support for funeral expenses and other means-tested benefits are available for cohabiting couples.”

Creasy gave the example of a constituent called Ros, whose husband died in 2014, and who had given up work earlier to support him during his illness. She said the mother of two had found the allowance to be a “lifeline” but would have lost out on more than £100,000 during the lifetime of her children as a result of the changes.

A second case was Joanna with two daughters, one born after the woman’s partner, David, suddenly died.

“They were clearly in a loving relationship. They had been together for a long period and had chosen not to be married. That should surely be their choice,” she said, pointing out that Joanna had not received the allowance, and had to pay £1,500 for a DNA test to include her partner’s name on her daughter’s birth certificate.

Creasy told the Guardian that the system was deeply unfair given that cohabiting couples were penalised during their lives. “If you are alive we would see you as a couple and tax you accordingly, but if your partner dies all his contributions get taken back by the state at a time when you need the financial support the most,” she said. She added that Harrington’s suggestion about people readjusting was “extraordinary”.

“It is a bizarre excuse to make for penalising widows. It just doesn’t make any sense,” she said.

Frank Field, the chair of the work and pensions committee, who triggered the parliamentary debate and has already persuaded the government to extend the time limit from one year to 18 months, said he was worried about the changes. He has written to the DWP to stress that ministers had originally claimed the reform was not about saving money.

During the debate Harrington also addressed the issue of support for people with funding funerals. He said the government would not tell people what sort of funerals were right, but highlight a “number of low-cost alternative options” that were emerging, including direct crematoriums which do not provide a funeral service.

“We recognise that those are not geographically widely available yet, and are not relevant to all religious and cultural practices. However, when it is appropriate, the industry should signpost people to direct cremation schemes and other low-cost alternatives so that bereaved people know they have the choice.”

Harrington, a pensions minister, was replacing another minister in the debate who was attending a funeral at the time.