"From Europe we have the umpteenth unacceptable interference by unelected officials", Salvini said.
The two parties have held six days of talks aimed at putting together a coalition government and ending 10 weeks of political stalemate following an inconclusive election on March 4.
"We need to be able to speak with a single voice, to say to the European Union, to which we pay many billions of euros every year, that for us Italians come first", the League's economics chief Claudio Borghi said, echoing US President Donald Trump's rallying cry "America First". "The European Union would not be complete without the Italian nation", Juncker said. If the agreement is passed, 5-Stars Luigi Di Maio said the details will be distributed over the weekend.
Salvini said the League wanted "strong government and a tax and security revolution", but admitted it remained "far apart" from M5S on several issues.
This will now be something to watch as populism is back on the agenda for markets where the coalition has radical ideas to free up billions of euros for tax cuts and welfare.
They have both vowed to scrap an unpopular pension reform - a move that would punch a 15-billion-euro hole in state coffers.
Salvini accused Brussels of meddling with the negotiations following a statement by Migration and Home Affairs Commissioner Dimitris Avramopoulos, who expressed the hope that Italian policy on asylum seekers will not change.
Salvini and Di Maio have both agreed to drop their own ambitions to be prime minister and are looking for a candidate from outside their parties to enact their program. 5-Star has moderated its position considerably in the past year, rowing back on a previous plan to hold a referendum on Italy's membership of the currency bloc.
They also share an anti-establishment sentiment that has taken root in Italy but which has global parallels such as Britain's vote to leave the European Union and the USA election of President Donald Trump. The policy program will probably be published Thursday, 5-Star said.
Europe offers no guarantees but vows to keep Iran deal alive
But Secretary of State Mike Pompeo said Washington was keen to thrash out a more wide-ranging deal with its European partners. "It is going to be very hard for us to preserve the economic benefits of the Iran deal", one senior European diplomat said.