Donald Tusk is to leave the prime ministership of Poland to become president of the European Council in succession to Herman Van Rompuy.

His appointment balances the choice of Federica Mogherini, Italy’s foreign minister, as the European Union’s high representative for foreign policy. The decisions were announced by Van Rompuy, who said that they had “the full support of all” members of the European Council.

“You have the full endorsement of the European Council and this full endorsement was very important to me,” said Van Rompuy, adding that he had spoken several times to each government leader “to make sure we made the right choice”.

The announcement came two and a half hours into an EU summit that was convened specifically to discuss the appointments – after Van Rompuy deemed the matter not ripe for decision at an earlier summit on 16 July. Agreement on Tusk and Mogherini was apparently painless, and the greater work of the summit may prove to be discussion of how to respond to Russia’s incursions into Ukraine.

Tusk, who has been prime minister since 2007, will take up his EU post on 1 December for a two-and-a-half year term. Mogherini will take up her post – which also includes a vice-presidency of the European Commission – when the new Commission to be led by Jean-Claude Juncker takes up office, which is scheduled to be on 1 November for a five-year term.

At a press conference at which Van Rompuy presented the two anointed EU leaders to the television cameras, Tusk addressed directly questions about whether he has the linguistic skills necessary for the Council presidency, announcing (in English) that for the moment he would answer questions in Polish, but in December he would speak in English. “Nothing is good enough for Europe – including my English. I will polish my English. I will be ready in December – 100%.”

He deflected a question from a Polish journalist about what should happen at the top of the Polish government, saying “we will not bore others about it” and adding that he would be ready to speak about it next week in Poland. Elections are scheduled for October next year.

Mogherini was questioned about the charges that she lacked experience and that her candidacy had been supported by Vladimir Putin, the Russian president. She rejected the first charge, responding that she was the foreign minister of a G7 country and that she had been involved in European and foreign affairs for 20 years. She stressed her experience of parliamentary institutions and of politics.

On alleged sympathy towards Russia, she said that her first trip abroad after Italy took on the presidency of the EU’s Council of Ministers had been to Kiev. After two days there, she had gone on to Moscow. She had been trying, she said, “to co-ordinate efforts to facilitate a form of dialogue”. She acknowledged that it was still not clear whether those efforts would bear fruit. “We will have time to discuss whether these ways have found any success, but in these hours it seems it has not.” “As we work on sanctions, we have to keep a diplomatic way open,” she said.

Tusk said he was confident that he and Mogherini would work well together. “I am very serene about our co-operation,” he said.

Van Rompuy, in his opening remarks, had identified three challenges that the new leaders of the EU would have to address: the EU’s stagnant economic performance; the Ukraine-Russia crisis; and “Britain’s place in Europe”.

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Tusk picked up this reference to the UK, saying that he had discussed with Prime Minister David Cameron his ideas for reform of the EU.

“No reasonable person can imagine the EU without the UK and I cannot imagine it myself,” he said. He also stressed that he did not want divisions to open up between the eurozone and the rest of the EU. As well as being president of the European Council, Tusk will be president of the euro summit – the meetings of government leaders  of the eurozone – a post that was given formal recognition by the 2012 fiscal compact treaty. Such eurozone summits are to happen twice a year and some meetings may be attended by all countries that are signatories to the fiscal compact treaty (currently 25 member states). That Tusk comes from a country that does not belong to the eurozone was not a barrier to his taking the euro summit presidency, despite some earlier objections from France.

Tusk presented himself as an unabashed pro-European. “I come to Brussels from a country that deeply believes in the significance of a united Europe. 80% of Poles believe in the EU and do not seek an alternative to the EU. I am also convinced that there is no intelligent alternative to the EU.”

The allocation of jobs means that the centre-right European People’s Party has the presidency of the European Council and the presidency of the Commission (Tusk and Juncker), while the centre-left Party of European Socialists (PES) has the post of foreign policy chief (Mogherini). The choice of Tusk reflects the preference of the leading figure in the EPP, Angela Merkel, Germany’s chancellor. She issued a statement of congratulation describing him as “a passionate European”. The choice of Mogherini reflects the preference of Matteo Renzi, suggesting that the Italian prime minister is the dominant figure in the PES.

Helle Thorning-Schmidt, the centre-left prime minister of Denmark, who had been talked about as a contender for the Council presidency, issued a statement congratulating Tusk. “He is an experienced politician who has contributed a lot to Poland,” she said. “He will be a good chairman of the European Council, and he will listen to every member. For Denmark it is important that there is support from every country – which Tusk has – and that is why Denmark is also supporting him.”