ROME (Reuters) - Italian Prime Minister Matteo Renzi on Wednesday threatened to block the European Union's next budget with a "veto" should Eastern European countries fail to take in their share of refugees.

"If the Eastern countries who are growing with the help of our money do not open their doors to migrants as has been agreed, then we'll put a veto on the future European budget," Renzi said at a rally in Avellino, Italy, for an upcoming referendum over his flagship reform of the constitution.

The proposal for the next EU budget, for the 2021-27 period, is supposed to be completed by the end of next year. Technically, Italy does not have a veto over the budget, which can be approved with a qualified majority vote, though unanimity is often sought.

The prime minister has frequently denounced a lack of European solidarity amid the migrant crisis as Italy seeks EU approval for an expansionary budget that includes some 3.9 billion euros ($4.25 billion) in spending on migrants next year, and as he campaigns for a referendum on which he has staked his future.

(Reporting by Steve Scherer and Isla Binnie)