Relate found 12 per cent of people stuck indoors with their partner were re-evaluating their future together, jumping to 21 per cent among those aged 25 to 34.
A survey of 2,021 UK adults revealed 22 per cent of men and 31 per cent of women are finding their partner irritating at the moment, and that almost a quarter of couples were having more rows than normal.
Money woes caused by the shutdown were a factor, with over a quarter of respondents saying cash was causing more problems than usual.
Parents with children living at home were also finding things tough, with 34 per cent saying it was difficult to create structure for them, and 24 per cent saying children's behaviour had become an issue.
Aidan Jones, chief executive of Relate, said: “We always see a peak in people seeking relationship support after Christmas, when spending unusually long together brings issues to the surface.
“Add to that the current extended period of isolation, worries about job security, finances, how to juggle work with childcare and uncertainty about the future - and it's clear why we're expecting a post-lockdown relationship reckoning.”
He urged people to consider webcam or telephone counselling before their relationship reaches crisis point.
Mr Jones said: “People coming to us for support are saying that the Covid-19 pandemic and its repercussions are magnifying existing issues.
“Everyone's trying their best to get through whilst stuck under one roof but that door won't stay closed forever, which is why we're urging anyone experiencing issues to get in touch now rather than letting things fester until things get irreparably bad.”
The charity said it was important to note that 65 per cent of those in relationships said they felt supported by their partner, while 43 per cent said lockdown had brought them closer together.
The extra time together did not necessarily translate into greater intimacy, with only 17 per cent of couples saying they were having more sex than usual.