Brussels said it hoped for "new beginnings" but made clear its position on Brexit "has not changed one iota."
Brussels and London are still tussling over Brexit but as Liz Truss became the new British Prime Minister on Monday, EU leaders chose instead to focus on shared values and the "challenging times" the two sides face.
In her victory speech, Truss praised her predecessor, Boris Johnson whom she said "got Brexit done" and "stood up to (Russian president) Vladimir Putin".
And Russia's war in Ukraine, as well as its consequences, appear to be what EU leaders most focused on as they reacted to Truss' victory.
Lithuanian President Gitanas Nausėda was among the first EU leaders to congratulate her, writing on Twitter that he wishes her "success and strength in leading the UK in the challenging times."
"The UK is Lithuania's trusted security partner and ally. Appreciate our cooperation in NATO and the UK-led JEF (Joint Expeditionary Force)."
Krišjānis Kariņš,Prime Minister of Latvia, which shares a border with Russia, said his country "highly values UK's outstanding contribution and leadership providing assistance to Ukraine."
"We will continue to stand together for Ukraine and our core democratic values," he also wrote in his congratulatory tweet.
German Chancellor Olaf Scholz said he is "looking forward to our cooperation in these challenging times. The UK and Germany will continue to work closely together - as partners and friends."
Roberta Metsola, the President of the European Parliament, meanwhile described the EU and UK as "natural friends and allies, with same core interests at heart."
"Democracies must remain united, in standing against autocracy and aggression," she added.
Slovak Prime MinisterEduard Heger emphasised "this time of challenge and global uncertainty" and said he hopes for a continuation "of strong UK leadership in European and global security as well as partnership with the EU."
In Prague, which currently holds the rotating presidency of the EU, Prime Minister Petr Fiala said he believes "that Great Britain will continue to be a valuable ally of the Czech Republic and European Union."
Yet, the spectre of Brexit still loomed over some of the congratulatory messages.
Irish Taoiseach Micheál Martin wrote for instance: "I look forward to working with you, as PM of our nearest neighbour, on important issues we face together, both bilaterally and globally."
Commission chief Ursula von der Leyen said for her part that she looks forward "to a constructive relationship, in full respect of our agreements", stressing that "the EU and the UK are partners" and together face "many challenges".
The UK's departure from the bloc, formally completed more than two and a half years ago, remains a painful thorn in EU-UK relations.
Brussels has launched several infringement procedures against London for not fully implementing the Northern Irish Protocol which kept the British province within the bloc's Single Market to prevent a hard border on the Irish island.
This created a de-facto border between the region and the rest of the UK which London immediately decried after formally backing the international treaty.
The UK must provide an answer to the EU's first round of infringement procedures by 15 September at the latest.
Commissioner Maroš Šefčovič, in charge of Brexit negotiations, said as he congratulated Truss, who until now was foreign, and Brexit minister, that he stands "ready to work intensively and constructively with my new interlocutor to foster such a partnership, in full respect of our agreements."
Earlier, Eric Mamer, the Commission's spokesperson in chief, had told reporters that the bloc is "always looking for new beginnings, obviously. Anything that can help move forward in our relationship with the UK will be very welcome."
"As to what we expect, it is extremely clear, it has not changed one iota, we expect the full implementation of the Withdrawal Agreement, the Trade and Cooperation Agreement that we have signed and, of course, the Northern Irish Protocol," he added.
Truss, 47, was in the Remain camp in the lead-up to the 2016 Brexit referendum but progressively aligned herself in the years since with the hard-Brexit fringe of her party. In her role as the UK's top diplomat she repeatedly made allusions to Article 16 to unilaterally pull out of the Northern Irish Protocol.
She was crowned leader of the ruling Conservative party following a two-month leadership contest that saw her triumph over seven other candidates.
She won the final round with 81,326 votes while runner-up Rishi Sunak garnered 60,399 ballots from the Conservatives' roughly 200,000 members.
She now succeeds Boris Johnson who was forced to step down on July 7 after losing the confidence of his MPs over a series of sleaze and COVID rule-breaking scandals.