Computer modelling compared actual temperatures with those that would have existed without human action.
The extreme temperatures hit agriculture and infrastructure particularly hard and people were urged to cut their water usage.
New computer modelling analysis compared the actual climate with that of the natural climate we would have had without human-induced emissions.
It found that the UK now has around a 12% chance of summer average temperatures being as high as they were in 2018, whereas they would have less than 0.5% chance of happening in a "natural" climate.
This summer was the equal warmest in a series dating back to 1910, along with 2006, 2003 and 1976, with temperatures reaching a peak on 27 July when 35.6C (96F) was recorded at Felsham, Suffolk.
Professor Peter Stott, from the Met Office and University of Exeter, said: "Our provisional study compared computer models based on today's climate with those of the natural climate we would have had without human-induced emissions.
"We find that the intensity of this summer's heatwave is around 30 times more likely than would have been the case without climate change."
The Met Office findings will be announced at the UN climate talks currently taking place in Poland, where countries are meeting to finalise the rules of how the Paris Agreement on tackling global warming will work and to build momentum towards increasing ambition on efforts to cut greenhouse gas emissions.
Greenpeace UK executive director John Sauven said: "The link between climate change and extreme weather like the heatwave that scorched the UK last summer is getting stronger.
"It used to be a fingerprint, it now looks more like a smoking gun. If we stay on the current course, we know the kind of world we're heading towards: more floods, heatwaves, droughts, and rising sea levels."