The UK’s carbon emissions have begun to rebound following the easing of Covid-19 lockdown measures, causing the "carbon savings" triggered by the coronavirus to halve within weeks.

Greenhouse gas emissions from the energy and transport industries climbed last month as more people returned to work, raising demand for fossil fuels from record lows in April when strict lockdown measures were in place, according to new data.

An analysis by Sia Partners, seen by the Guardian, shows that the UK’s carbon emissions fell by 36% in the first four weeks of the lockdown compared to the most recent official carbon emissions data collected in 2018.

But by June Britain’s total emissions savings had dwindled to a 16% drop as more cars returned to its roads and demand for energy began to rise.

Chloé Depigny, a senior manager at Sia Partners, said the data underlines the fragility of the UK’s short-term carbon savings during the coronavirus, and the need for ambitious fundamental changes to the economy if the government hopes to meet its long-term carbon targets.

The data reveals that at the start of the lockdown Britain recorded a 90% collapse in carbon emissions from the aviation sector, a 60% fall in emissions from passenger vehicles and a 30% decline in emissions from Britain’s energy system.

In the last month the emissions savings from road use shrank from 60% to 30% of typical levels, and the decrease from the energy system contracted from 30% to 15%.

If lockdown measures are removed entirely by early October the total carbon savings from the coronavirus may erode to 10% below normal levels over the year as a whole, down 1 percentage point from the consulting firm’s previous full-year forecasts earlier this year.

"In order to meet the UK’s net zero target by 2050 the UK needs to cut 12 megatonnes of CO2 every year – this is the equivalent of 3% of the emissions in 2018. So 10% is definitely a significant fall," she said.

"However, from a climate point of view if this only occurs in 2020 and normal emissions return in 2021 then these savings will mean only a very small dent to emissions in the end. We saw this in the 2008 financial crisis; emissions very quickly returned to pre-crisis levels," she added.

One of the greatest threats to the UK’s carbon savings this year is a surge in demand for road travel as more people opt to use passenger vehicles over public transport to avoid contact with the coronavirus.

"This is one of the big uncertainties as we emerge from lockdown," said Depigny. "If everyone is concerned about using public transport, and chooses to switch to using cars, then road emissions may well explode over the second half of the year, and could cause even a 10% emissions cut to disappear within a few months."

Another threat to carbon reductions in the wake of the coronavirus outbreak is the number of people who may continue to work from home during the winter months.

The report predicts that the carbon footprint of British homes is expected to be 6% higher than normal in 2020 based on the assumption that many people working from home will make a gradual return to office spaces from October. But a second lockdown during the colder months could mean far higher residential carbon emissions than currently forecast, Depigny said.

"We have been lucky that lockdown has happened during warm summer months. If there was a second lockdown during the winter, homes would rely on gas-heating to keep their homes warm all day, which would produce far more emissions than during the summer. It would probably counter most of the savings from less commuting," she said.