The study, based on a survey of 20,980 people in relationships from 2013-15, suggested that as many as 2.87 million people (around 18% of married or cohabiting couples) could be in a relationship on the verge of breaking-up.
Almost half of respondents (49%) admitted to bickering, while 6.8% reported severe levels of arguing within the relationship. Meanwhile, one in 10 partners admitted at least occasionally regretting getting married or living together, while 9% said they at least occasionally considered divorce or separation.
The pressure of jobs, finances and childcare were found to be flash points which caused the most arguments.
Those with children under 16 were slightly more likely to be in distressed relationships, and the report found that becoming a first-time parent was "one of life's events most likely to reduce relationship quality."
Interestingly, the study also revealed that while money could make marriages more stable, it could also make couples more likely to get divorced; the study found that the number of people in troubled relationships peaked around the time of the last recession but has since fallen, suggesting that financial instability was putting a strain on relationships.
However, the number of respondents considering divorce or separation increased following the recession, pointing to the conclusion that couples were more likely to consider ending a relationship if there finances were more secure.
Dr David Marjoribanks, senior policy officer at Relate, said: "There is a pattern of relationship strain increasing during recession years, and that is very much what we would expect.
"Where economic strain increases, for example low income, unemployment, a build up of debt, the strain on the relationship increases.
"But the level of relationship distress hasn't gone back to pre-recession levels, so while we might be out of the woods in terms of recession the impact on relationships clearly doesn't seem to have completely gone."
He also warned that many couples suffered in silence for years and only sought help when it was too late to salvage their relationship.
Chris Sherwood, chief executive at Relate, called the report's findings "hugely concerning", and warned that "families can't go on like this".
Following the report, Relate is launching Breaking Point, a national appeal to raise money through donations to help make its services available to all couples, regardless of their financial position.