First reported by openDemocracy, the document dated last month suggests officials are considering cutting aid to Syria by 67% and Lebanon by 88%.
Aid to Nigeria could drop 58%, Somalia 60%, South Sudan 59% and the Democratic Republic of Congo by 60%, it says.
The Foreign Office said no decisions had been made and insisted the UK remained a world-leading aid donor.
Syria has been devastated by a conflict that erupted after President Bashar al-Assad’s government responded with deadly force to peaceful anti-government protests in March 2011.
The fighting has left at least 380,000 people dead and caused half the population to flee their homes, including almost six million refugees abroad.
A UK government spokesman said: “The seismic impact of the pandemic on the UK economy has forced us to take tough but necessary decisions, including temporarily reducing the overall amount we spend on aid.
“We are still working through what this means for individual programmes and decisions have not yet been made.
“We remain a world-leading aid donor and we will spend more than £10 billion this year to fight poverty, tackle climate change and improve global health.”
The government wants to reduce its overall international aid budget by about £4bn in 2021-2022, which will mean missing the UN target of spending 0.7% of national income on foreign aid.
But it has yet to say where the axe will fall.
The leaked Foreign Office email gives the first indication of which countries might lose out.
And the figures suggested by the document are of the same scale as the 50% cut in aid to Yemen announced earlier this week.
The UK has pledged at least £87m in aid to Yemen, down from £164m in 2019-2020.
Kevin Watkins, chief executive of Save the Children, said the UK’s decision to cut aid would have “devastating real-life consequences”.
“Across the affected countries, 105 million children are living in high-intensity conflicts – two thirds of the global total,” he said.
“We are looking at the near collapse of UK help for children trapped in the world’s worst war zones, just as a second wave of the pandemic bears down on many of them.”