Greeks owed the state 97.3 billion euros in unpaid taxes in July, some 55% of the country's gross domestic product, two billion euros higher than the previous month, Finance Ministry data showed Monday.
The spike in unpaid taxes comes after the left wing government rolled out the latest round of austerity measures needed to clinch rescue funding for the country from international creditors - eurozone nations and the International Monetary Fund.
The belt tightening measures come amidst a slower than expected recovery in the economy.
Greece's economy was expected to rebound strongly in 2017, helped by the boom in tourism, but growth has been sluggish due to higher taxes imposed on businesses and unemployment rates staying above the 20% mark.
The spike in unpaid taxes means that up to 1.6 million taxpayers in Greece - a country of nearly 11 million people - face the risk of the tax office confiscating their assets, according to some estimates from government officials.
In an attempt to recover these outstanding amounts, Greece's revenue collectors have been confiscating assets, such as bank deposits and real estate, and incomes from about 1,000 tax payers per day.