"The economy will decline by around 10% this year," Theodore Skylakakis told Proto Thema radio. He said public debt was likely to rise to around 204%-206% of gross domestic product as a result.
The projection means Greece's recession-hit economy could shrink at a rate closer to its central bank's worst-case scenario, a 9.4% GDP decline.
Last month, the Bank of Greece said the economy was likely to contract by 7.5% this year under a baseline scenario, faring worse than a previous forecast of a 5.8% decline.
In its draft 2021 budget released in early October, the government projected the economy would shrink 8.2% this year and rebound by 7.5% in 2021.
It projected public debt would decline to 184.7% of GDP in 2021 from 197.4% this year.
Greece ordered a nationwide lockdown for three weeks earlier this month, its second this year, to help contain a resurgence of COVID-19 infections.
Prime Minister Kyriakos Mitsotakis said he was forced to act as pressure on the country's healthcare system would be "unbearable" without a lockdown.
(Reporting by George Georgiopoulos; Editing by Steve Orlofsky)