Luigi Di Maio, leader of the Five Star Movement (5SM), could become the next Italian Prime Minister if the votes go his way next month.
And he is now urging the EU to thrash out a good Brexit deal in both the bloc and Britain’s interests.
He has previously hinted his party could hold a referendum on the EU if there are not fundamental changes to the way it was run.
And he hit out at the EU again today over its treatment of Britain, saying member states would suffer too if Brexit ended badly.
Mr Di Maio said he did not believe Britain should be punished for leaving the bloc but stressed Brussels must safeguard its investments.
He told RT: “Our interest is to first of all protect the hundreds of thousands of Italians currently living in the UK.
“There was one point where the EU wriggled out of this conviction whereby the UK should be punished because of Brexit. I don’t think it should be punished.
“I think there is a single market, and there are advantages for who is in the EU, and disadvantages for who isn’t in it.
“We must safeguard the investments and give continuity to the economic relationships with the EU of the past years.
“This must also be the case for the UK.”
Mr Di Maio’s comments come in the same week he demanded fundamental changes in the bloc and said Brexit had “weakened” the EU.
And he told CNBC that holding an EU referendum would be his “last resort” as leader, should reform prove impossible.
Mr Di Maio said: “Germany, France Spain are renegotiating some of the EU rules.
“This is the time to make some deficit, to make investments and re-launch the Italian economy.
“I don’t want to even consider that last resort – I believe in the power of dialogue.
“Other European countries are also weaker; Germany can’t form a government, in France traditional political parties don’t gain much traction, Spain and Portugal have minority governments.
“And, of course, there is Brexit.
“Brexit weakened the EU. We need to re-negotiate some EU rules, but not in an in/out referendum.”
He also predicted his party would be the largest in the Italian parliament after the March elections.
There has been talk the party could form a coalition with the similarly Eurosceptic Northern League, causing a nightmare for Brussels.
Any referendum on Europe has the potential to throw the single currency into chaos, as Italy is the eurozone’s third-biggest economy, behind Germany and France.
Last month, Northern League leader Matteo Salvini went on a foul-mouthed rant against the bloc, declaring: “Europe can go f*** itself.
“Europe has been punishing us for the last 15 years and we are worse off than 15 years ago.
“European measures are the last thing I am interested in.”