Guy Verhofstadt, chief Brexit representative for the the European Parliament, criticised Mr Juncker for publishing a letter he sent to Mr Putin, referring to him as “excellency, Mr President” and calling for “positive relations” – despite the UK urging EU allies for a tough line on Russia.
The public dispute comes after it emerged that the UK will not push for sanctions on Russia at a European Council summit later this week, with there thought to be little appetite among other member states.
Reports also emerged that European foreign ministers had toned down language criticising Moscow at the behest of sceptical EU members.
The news left doubt over whether Theresa May can win a tough joint statement of support from EU leaders at a council summit on Thursday, despite having already won over France and Germany.
It comes as the UK continues to seek broad international condemnation of Russia over the attack, which saw a military-grade, Russian-made nerve agent released in Salisbury on 4 March – leaving ex-spy Sergei Skripal, his daughter and a police officer in hospital.
After Mr Juncker – the most senior official in the EU – published his letter on Tuesday morning congratulating Mr Putin on his election win, Mr Verhofstadt took to Twitter to attack the move.
The former Belgian PM said: “This is no time for congratulations.
“We will always need dialogue with Russia, but closer ties must be conditional on respect for the rules based international order and fundamental values.”
Mr Juncker posted his letter on social media, just Britain is attempting a round of diplomacy to build a coalition for a hard line against Moscow and throwing Russian diplomats out of the UK in response to the Salisbury incident.
The note started “Excellency, Mr President”, before conveying congratulations and continuing: “I have always argued that positive relations between the Russian Federation and the European Union are crucial to the security of our continent.
“Our common objective should be to re-establish a cooperative pan-European security order.”
Urging Mr Putin to use his fourth term to pursue that goal, he added: “I will always be a partner in this endeavour”.
The letter will also likely rile European Council President Donald Tusk, who is understood to be keen to attribute blame to Russia, though the decision will lie with member states.
A senior EU official said: “As far as I know President Tusk hasn’t sent such a letter [like Mr Juncker’s] until now.
“I would not be surprised if he would not send one at all.”
The EU official confirmed the UK had not approached Brussels “with any request to put on the table restrictive measures” on Russian trade.
Ms May is instead expected to press leaders at this week’s summit for a “common line on attribution” to blame Russia for the Salisbury incident, as well as “a need to ensure coordination for the future” – but even that could be challenging.
Foreign ministers who met on Monday agreed that the EU takes the UK Government’s assessment that it is highly likely Russia is responsible “extremely seriously”, but did not blame Moscow outright.
A failure to get a united statement attributing blame to Russia would be a boost for Mr Putin, who until this point had faced a joint front from France, Germany, the US, Nato, Canada, Australia and other nations, though Donald Trump today also congratulated Mr Putin on his win.
Join our commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies