Matteo Renzi, political rival of Italy's Prime Minister Enrico Letta, has called for a "new government".
Mr Renzi, 39, is the leader of Mr Letta's Democratic Party and made the remarks during a party meeting in Rome.
There have been days of speculation that Mr Renzi wants to take over the post of prime minister.
If Mr Renzi's proposal is approved by the party leadership, analysts say Mr Letta may have to resign as early as Thursday evening.
There is a growing sense that the party is likely to back Mr Renzi, the BBC's Alan Johnston reports from Rome.
Relations between the two men have become increasingly fraught, our correspondent adds.
Way out of 'morass'
Mr Letta, 47, formed a coalition with the centre-right last year but there had been mounting speculation over his future after Mr Renzi was elected party leader in December.
Mr Letta has reportedly cancelled a trip to the UK scheduled for later this month.
He had previously said he was unwilling to resign.
"You don't step down because of gossip, because of power plays and behind-the-scene activities," the prime minister said hours after a face-to-face meeting with Mr Renzi in Rome on Wednesday.
Mr Letta did not attend Thursday's party meeting, which had been brought forward from next week.
Mr Renzi thanked Mr Letta for his "remarkable work" but said the country could not go on in "uncertainty".
He said his proposal - a new government to take over until the end of the current parliamentary term in 2018 - was a way out of the "morass".
He has previously accused Mr Letta of a lack of action, with unemployment at its highest level in 40 years and the economy shrinking by 9% in seven years.
Correspondents say that although the prime minister has kept a careful lid on public expenditure, he has not been able to carry out much-needed administrative reforms and stimulate economic growth.
The latest political turmoil has so far had little impact on financial markets, in contrast with the volatility seen before the current coalition was formed.