A €2 billion ($2.1 billion) deal was agreed to by EU ministers on Monday to restock their own arsenals and jointly purchase ammunition for Ukraine.
Defence and foreign ministers backed an initiative aimed at providing Ukraine with one million artillery shells in the next 12 months as well as replenishing EU stocks.
Russia's year-long invasion has turned into a grinding war of attrition and Ukraine has complained that its forces are having to ration firepower.
Ukraine had told the EU it wants 350,000 shells a month to help its troops hold back Moscow's onslaught and allow them to launch fresh counter-offensives later in the year.
German Defence Minister Boris Pistorius said the “starting signal” had been given and promised Kyiv could count on getting a large amount of ammunition this year.
Ukraine's top diplomat Dmytro Kuleba hailed the “game-changing decision” from the EU.
“Exactly what is needed: urgent delivery and sustainable joint procurement,” Mr Kuleba wrote on social media.
The first part of the EU plan commits a further €1 billion ($1.06 billion) of shared funding to try to get EU states to tap their already stretched stocks for ammunition that can be sent quickly.
The second part will include the bloc using another €1 billion to order 155mm shells for Ukraine as part of a massive joint procurement push intended to spur EU defence firms to ramp up production.
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Buying weaponry together on this scale is a major new step for the EU, which has seen long-standing efforts to work more in unison on defence, propelled by Russia's war.
Countries wrangled over details, like whether it should be the EU's defence agency or the member states that negotiate the orders and if they should buy only from producers in Europe.
EU foreign policy chief Josep Borrell said the plan was to purchase from European manufacturers and sign contracts for the ammunition for Ukraine by the end of May.
But Lithuanian Foreign Minister Gabrielius Landsbergis warned that the ambition to supply one million rounds over the next year was not set in stone.
“It is possible that we might not be able to reach it,” he admitted.
After 12 months of eating into their stockpiles, there are questions over how much EU countries can share immediately without leaving themselves vulnerable.
The bloc has already committed a wide range of military support worth €12 billion to Ukraine, with €3.6 billion from a joint fund used to help cover the costs.
Officials say that, since the Russian invasion last February, €450 million from the fund have been used to supply 350,000 shells to Ukraine.
Key to getting countries to deplete their stocks is convincing them that European industry can step up to produce more.
Ukraine's consumption of ammunition currently far outstrips the amount its western backers are manufacturing.
Brussels said EU firms need to switch to “war economy mode” after scaling back in the years following the end of the Cold War.
The industry complains that governments have not yet signed the long-term contracts they need to invest in more production lines.
The EU is hoping that placing a mammoth joint order for 155mm shells will incentivise companies to ramp up their output.
But there are concerns about the supplies of key components such as explosives.
“We are used to production that is for the peacetime and this time we are in war in Europe,” Finnish Foreign Minister Pekka Haavisto said.
“This will take time of course; there is also some lack of the materials and so forth.”
EU internal market commissioner Thierry Breton visited French firm Nexter on Monday as part of a tour of 15 ammunition firms in 11 EU countries aimed at urging them to push ahead.
He said Brussels was ready to intervene, including with central funds, to help companies increase production.
“Increasing industrial capacity is essential,” he said.
