By applying a novel research method - Mendelian randomisation - which uses genetic and surveys data from hundreds of thousands of people, the team found that loneliness appears to lead to an increased likelihood of smoking behaviour.
"We found evidence to suggest that loneliness leads to increased smoking, with people more likely to start smoking, to smoke more cigarettes, and to be less likely to quit," said co-lead author Robyn Wootton from the University of Bristol in the UK.
There was evidence that being lonelier increases the likelihood of starting smoking, the number of cigarettes smoked per day and decreases the likelihood of successfully quitting.
This reflects the trends observed during the pandemic. YouGov's Covid-19 tracker suggests 2.2 million people across the UK are smoking more than they were before lockdown.
In the other direction, there was also evidence that starting smoking increased individuals' loneliness.
"Our finding that smoking may also lead to more loneliness is tentative, but it is in line with other recent studies that identified smoking as a risk factor for poor mental health," said study senior author Dr Jorien Treur.
The team also investigated the relationship between loneliness and alcohol use and abuse and found no clear evidence for a causal relationship there.
According to the Office of National Statistics (ONS), during the first month of the lockdown, the equivalent of 7.4 million people said their well-being was affected through feeling lonely.
Lonely people were more likely than others to be struggling to find things to help them cope and were also less likely to feel they had support networks to fall back on, the study said.
"We are still yet to see the full effects of the coronavirus pandemic on alcohol and cigarette use in the UK," Wootton said.
This research highlights the need for smokers suffering from loneliness to be given support to stop, to improve not just their health and wellbeing but also to help reduce their loneliness.