Volodymyr Zelenskiy has taken aim at the Hungarian prime minister, Viktor Orbán, accusing him of betraying fellow European leaders after Orbán’s recent “peace mission” to Moscow.

The Ukrainian president kicked off the European Political Community summit at Blenheim Palace on Thursday with an emotional speech in which he made veiled but repeated references to Orbán’s recent attempts to get close to Moscow.

Zelenskiy was addressing a room packed full of European leaders and defence ministers, who have gathered in Oxfordshire at a pivotal moment for the war in Ukraine.

He told the session: “We have maintained unity in Europe by acting together, which means that Putin has missed his primary targets … This is our advantage, but it remains an advantage only as long as we are united.”

Referring to Putin, he said: “He may try to approach you, or go to some of your partners individually, trying to tempt or pressure you to blackmail you so that one of you betrays the rest. We keep our unity.”

In an apparent reference to Orbán’s recent visit to meet Putin in Moscow, he added: “If someone in Europe tries to resolve issues behind our backs, or even at the expense of someone else, if someone wants to make some trips to the capital of war to talk – and perhaps promise something against our common interests or at the expense of Ukraine or other countries – then why should we consider such a person?

“The EU can also address all their issues without this one individual.”

Orbán has regularly been a thorn in the side of European leaders attempting to maintain unity in their support for Ukraine. Earlier this year he blocked a €50bn (£42bn) support package for Kyiv for more than a month while he negotiated additional conditions.

This month, the Hungarian leader travelled to Moscow for what he called a “peace mission”, holding a joint press conference with Putin in which the Russian leader told Kyiv to give up more land, pull back its troops and drop its efforts to join Nato.

Orbán then held talks with the former US president Donald Trump, after which he wrote to European leaders telling them Trump had “well-founded plans” for peace should he return to the White House after November’s presidential election. Trump has since underlined his desire to back out of foreign conflicts by choosing the isolationist JD Vance as his running mate.

Orbán’s actions on the world stage have caused alarm in European capitals, with Zelenskiy’s invitation to Blenheim Palace designed in part as a show of unified support for Ukraine.

Hours before the Ukrainian president spoke, Ursula von der Leyen issued a similar message in a speech to MEPs. The president of the European Commission said: “Two weeks ago, a European prime minister went to Moscow. This peace mission was nothing but an appeasement mission.”

Keir Starmer, the British prime minister, made a strong statement of support as he introduced Zelenskiy on Thursday. “Every day Ukraine fights to protect not just the Ukrainian people, but the European people – a continent where our belief in freedom, democracy and the rule of law was hard won, that wants to live in peace,” he said.

“President Zelenskiy, in your struggle to uphold those values, we salute you. Once again, have no doubt, we will stand with you for as long as it takes.”

Zelenskiy also used his speech to European leaders to urge them to provide air defences for Ukraine and not to place limits on their use. The UK has provided Ukraine with Storm Shadow missiles, but is not allowing Zelenskiy to use them against Russian territory.

“We should not fear these capabilities,” the Ukrainian president said. “The more effective our air defences, the more helpless Putin will be. The fewer restrictions we have on the use of effective weapons, the more Russia will seek peace.”