The report, carried out by charity Relate and based on a survey of 20,980 people in relationships from 2013-15, suggested 2.87 million people were in "distressed" relationships.
Dr David Marjoribanks, from Relate, said constant bickering could have a "far-reaching" impact on children.
On average they did worse in school and could even fall into crime, he said.
"It is not just the actual breakdown of the relationship itself, it's specifically the conflict that surrounds that," Dr Marjoribanks said.
"It means that when relationships end, it is not deemed to inevitably harm children, far from it.
"It is the conflict in intact relationships that can be just as damaging, as when relationships end," he added.
"Children who grow up with parents who have highly-conflicted relationships are much more likely to have mental and physical health problems, to not do as well at school and end up in antisocial behaviour and criminality even."
It may have seemed a small thing but Sophie*, 28, knew her marriage was over when she came home from work to have lunch with her out-of-work husband and he expected her to make it.
Things had not been right for some time. There had been arguments about money, jobs and housework shortly after the wedding, often in front of their young son. But soon communication shut down almost entirely as both felt the arguments were always the same and nothing would ever change.
Days would go by with neither of them talking to each other.
They tried counselling but it came at a cost and they found themselves forced to choose between a counselling session or buying food for the week.
In January last year, after three years of marriage, Sophie told her husband, 41, it was over.
"As soon as we separated I immediately felt lighter. I did not have to do all this stuff for someone who did not do anything," she said.
Both are now in new relationships, and Sophie, who lives in West Yorkshire, says she now tries to make time to talk about things as soon as they come up and to be more open about money.
*Not her real name
Researchers looked at data from the Understanding Society survey of 20,980 people which asked people how often they argued, how frequently they considered divorce and regretted the relationship, and the extent of their unhappiness.
They said their findings suggested 2.87 million people, which equates to 18% of married or cohabiting couples, were living in "distressed" relationships, where the strains were deemed to be "clinically significant" by counsellors.
The number of "distressed" relationships reached a high in 2011 and 2012 but have not yet returned to pre-recession levels, the data showed.
"There is a pattern of relationship strain increasing during recession years - where economic strain increases, for example low income, unemployment, a build-up of debt, the strain on the relationship increases," Dr Marjoribanks said.
The research also found:
Dr Marjoribanks said many couples suffered in silence for years and only sought help when it was too late to salvage their relationship.
Jan Artingstall from Therapy Cheshire, who is listed in the Counselling Directory, believes people today spend so much time communicating via text and social media that they have forgotten how to talk.
"People have lost the art of talking about how they feel. It's like we have gone back to being children who don't have the language to communicate feelings," she said.
A common problem among couples was mistrust built around text messages sent between work colleagues and partners spying on each other's online communications, she added.
Her advice is for couples to sit down for 10 minutes to talk about their day and take joint responsibility for the state of the relationship, rather than pointing fingers.
She also said children who saw their parents fall out and make up were learning a useful lesson but daily and embedded conflict was damaging.
"Children are very perceptive to atmosphere. It doesn't have to be a shouting match - they can pick up on stonewalling and tense body language.
"Children won't say 'Are you unhappy Mummy or Daddy?' They just accept and absorb the atmosphere and feel unhappy inside."
There were 114,720 divorces in England and Wales in 2013, down 3% on 2012, the most recent figures available from the Office of National Statistics show. The number of divorces was highest among men and women aged between 40 and 44.
In Scotland, 9,030 divorces were granted in 2014-15, 6% fewer than in the previous 12 months, Scottish government figures show. Northern Ireland saw a slight rise in divorce rates from 2,403 in 2013 to 2,455 in 2014, according to the Northern Ireland Statistics and Research Agency.
Chris Sherwood, chief executive at Relate, said the report's findings were "hugely concerning", adding that "families can't go on like this".
The charity was launching its first national appeal, Breaking Point, calling for donations to help make its services available to everyone, not just those who could afford them, he added.
Almost one in five (18%) couples in the UK argue regularly or consider separating, a study suggests.