In an interview with the business daily Il Sole 24 Ore, Economy Minister Giovanni Tria said he now expected the country's gross domestic product to expand by 1.2 per cent this year, down from the 1.5 per cent forecast by the previous government.
And GDP growth next year was only set to come out at 1.0-1.1 per cent, instead of the previous prediction of 1.4 per cent, he said.
The Bank of Italy, the European Commission and the International Monetary Fund (IMF) have already lowered their forecasts for the eurozone's third-biggest economy.
Tria said that slower growth would likely inflate Italy's public deficit, which the previous centre-left administration had predicted would amount to 1.6 per cent of GDP in 2018 and 0.8 per cent in 2019, with a balanced budget projected for 2020.
Under EU rules, member states are not allowed to run up deficits in excess of 3.0 per cent of output.
"The slowdown alone would bring the deficit (ratio) to 1.2 per cent in 2019," Tria said.
"We are in dialogue with the European Commission," he added.
When Tria presents his 2019 budget in September or October, he will have to take great care not to scare financial markets, while at the same time appearing to uphold the anti-austerity pledges of the new populist coalition government, made up of the Five Star Movement and the League parties.
Tria said it was "in the interests of both Italy and the European Commission not to create financial instability".
But he insisted Rome would continue to bring down its deficit.
"There will be a slowdown compared to the trend forecast a few months ago. But what counts is that (deficit) reduction is not in question," he said.