Men are usually programmed to prefer slimmer and younger-looking women because they appear healthier and more fertile, making them a better bet for bearing offspring.
But stress can cause men to reassess their priorities, with panic making them treasure more homely qualities such as a larger body size which signify access to basic resources like food, scientists said.
Body fat is also linked to age, meaning a larger waistline may signify that a woman is more mature and therefore better equipped to cope with threatening situations, making her a more appealing partner in uncertain times.
The study backs up previous research which has shown that being hungry can cause men to prefer larger women, and that older-looking actresses become more popular during times of recession.
Researchers from Westminster University split 80 volunteers into two groups, with half undertaking a series of stressful activities such as mock job interviews and mental arithmetic tests to make them feel tense.
Both groups were then shown pictures of a range of different female body shapes ranging from emaciated to obese, and asked to rate them in terms of how attractive they were.
The results showed that men in the stressed group rated their "ideal" figure as significantly larger than those in the control group.
While both groups gave broadly similar ratings to slimmer women, the stressed men were also much more likely than men in the control group to find women who were of normal weight or overweight attractive.
Dr Viren Swami and Dr Martin Tovee wrote in the PLoS ONE journal: "While there was no significant difference in the lower end of the range, the [stressed] group appear to have shifted the maximum cut-off for attractive bodies at higher BMIs (body mass indices)."
The pictures were numbered according to their BMI on a scale of one to ten, with one representing emaciated and ten obese.
The largest figure deemed attractive by stressed men was 7.17, which fell in the "overweight" category, while the upper limit of attractiveness for men from the non-stressed group was 6.25, which is classified by the scale as a "normal" weight.
Stressed men rated a figure of 4.44 as the ideal figure while the non-stressed men found women most attractive at a lower level at 3.90.
"The results provide the first experimental evidence that the experience of psychological stress shapes men's judgements of female body size," the researchers wrote.
"Men experiencing stress not only perceive a heavier female body size as maximally attractive but also more positively perceive heavier female body sizes, and have a wider range of body sizes considered physically attractive."