Climate experts have warned that the United Kingdom could fall behind on greenhouse gas reduction targets if emissions are allowed to proliferate when COVID-19 lockdown is lifted.

Members of the UK's Committee on Climate Change, or CCC, have urged the government to keep sustainability in mind when making up for lost productivity brought on by the novel coronavirus outbreak.

John Gummer, chairman of the CCC, said that while the pandemic has caused the "biggest economic shock for a generation", it also provides nations with an "once-in-a-lifetime opportunity" to speed up global response to the climate crisis.

In its annual report to Parliament published on Thursday, the CCC said that steps taken by the UK to rebuild after the COVID-19 pandemic can accelerate the transition to a low-carbon economy.

Global emissions are expected to fall by 5 to 10 percent in 2020 due to reduced movement and industrial activity during the pandemic. But CO2 output is expected to rebound and may even surpass previous levels as nations emerge from lockdown.

"Choices that lock in emissions or climate risks are unacceptable," Gummer said.

The report found that while the UK reduced overall emissions by 3.2 percent in 2019, the country is only on track to fulfill four out of 21 indicators of progress on climate change identified by the CCC.

The majority of UK emissions reductions last year came from decarbonization in the power sector. Emissions from electricity generation fell by 67 percent between 2008 and 2019, the UK's biggest success yet on the road to net zero.

However, the CCC says that most other sectors are cutting emissions too slowly, and in some cases, emissions are on the rise.

Last year industries including surface transport and buildings showed modest decreases of between 1 and 2 percent, while those from shipping, aviation and waste all increased.

The CCC said that the UK risks missing its target of net-zero emissions by 2050 without improvement across multiple industries.

"The UK's domestic ambition can be the basis for strong international climate leadership, but the delivery of effective new policies must accelerate dramatically if we're to seize this chance," said Julia King, chair of the CCC's Adaptation Committee.

The CCC's key policy recommendations include: Low-carbon retrofits and buildings that are fit for the future; tree planting, peatland restoration, and green infrastructure; strengthening of energy networks; better infrastructure for walking and cycling; and moving toward a circular economy.

"We agree with the committee that tackling climate change should be at the heart of our economic recovery," a UK government spokesperson said in response to the report. "We believe that the actions we need to take to achieve our zero emissions target can help to deliver a stronger, cleaner, more sustainable and more resilient economy after this pandemic."