The British foreign secretary, Boris Johnson, has said he is encouraged by the response from friends willing to rally to the UK cause over the poisoning of the former Russian spy Sergei Skripal. But EU officials warned privately that the bloc’s existing consensus on further sanctions against Russia has already been cracking, and it may be impossible to forge collective agreement on new measures.

One official said the best that may happen is that the EU resolves to stick to its existing sanctions position rather than see it diluted.

Following a round of diplomatic phonecalls coordinated with Number 10, Johnson had been buoyed by support overnight from the US secretary of state, Rex Tillerson, only to find that Donald Trump had fired him after repeated run-ins.

Trump himself said on Tuesday that he would speak to Theresa May about the poison attack, after facing criticism for not swinging clearly behind the UK view that the Russian government was either responsible for the attack or for failing to control its nerve agent stocks. “It sounds to me that it would be Russia based on all the evidence they have,” he said. “As soon as we get the facts straight, if we agree with them, we will condemn Russia or whoever it may be.”

Johnson was also claiming support from the Nato secretary general, Jens Stoltenberg, and the French foreign minister, Jean-Yves Le Drian. Guy Verhofstadt, president of the European parliament and an advocate of conditional engagement with Russia, said: “We stand shoulder to shoulder with the British people.” Support also came from the European commission vice-president, Frans Timmermans.

No EU country has so far committed itself to further sanctions against Russia if the UK declares the Russian state had acted unlawfully on British soil.

Timeline Poisoned umbrellas and polonium: Russian-linked UK deaths Show Hide Georgi Markov In one of the most chilling episodes of the cold war, the Bulgarian dissident was poisoned with a specially adapted umbrella on Waterloo Bridge. As he waited for a bus, Markov felt a sharp prick in his leg. The opposition activist, who was an irritant to the communist government of Bulgaria, died three days later. A deadly pellet containing ricin was found in his skin. His unknown assassin is thought to have been from the secret services in Bulgaria. Alexander Litvinenko The fatal poisoning of the former FSB officer sparked an international incident. Litvinenko fell ill after drinking tea laced with radioactive polonium. He met his killers in a bar of the Millennium hotel in Mayfair. The pair were Andrei Lugovoi – a former KGB officer turned businessman, who is now a deputy in Russia’s state Duma – and Dmitry Kovtun, a childhood friend of Lugovoi’s from a Soviet military family. Putin denied all involvement and refused to extradite either of the killers. German Gorbuntsov The exiled Russian banker survived an attempt on his life as he got out of a cab in east London. He was shot four times with a silenced pistol. He had been involved in a bitter dispute with two former business partners. Alexander Perepilichnyy The businessman collapsed while running near his home in Surrey. Traces of a chemical that can be found in the poisonous plant gelsemium were later found in his stomach. Before his death, Perepilichnyy was helping a specialist investment firm uncover a $230m Russian money-laundering operation, a pre-inquest hearing was told. Hermitage Capital Management claimed that Perepilichnyy could have been deliberately killed for helping it uncover the scam involving Russian officials. He may have eaten a popular Russian dish containing the herb sorrel on the day of his death, which could have been poisoned. Boris Berezovsky The exiled billionaire was found hanged in an apparent suicide after he had spent more than decade waging a high-profile media battle against his one-time protege Putin. A coroner recorded an open verdict after hearing conflicting expert evidence about the way he died. A pathologist who conducted a postmortem examination on the businessman’s body said he could not rule out murder. Scot Young An associate of Berezovsky whom he helped to launder money, he was found impaled on railings after he fell from a fourth-floor flat in central London. A coroner ruled that there was insufficient evidence of suicide. But Young, who was sent to prison in January 2013 for repeatedly refusing to reveal his finances during a divorce row, told his partner he was going to jump out of the window moments before he was found.

Skripal poisoning Sergei Skripal and his daughter Yulia were were found unconscious on a bench in the Maltings shopping centre in Salisbury after 'suspected exposure to an unknown substance' which was later identified as chemical weapon novichok. In the aftermath Theresa May blamed Vladimir Putin and expelled 23 Russian diplomats who were suspected of spying. Two Russian men using the identities Ruslan Boshirov and Alexander Petrov were named as suspects. They appeared on Russian TV to protest their innocence.

The Skripals survived. However a local woman, Dawn Sturgess, died after spraying novichok on her wrists from a fake Nina Ricci perfume bottle converted into a dispenser, which had been recovered from a skip by her partner Charlie Rowley.

In a major test of UK diplomatic heft in a post-Brexit era, the UK has been putting out feelers to sense what countries would be prepared to do in a show of solidarity.

In a sign that some EU countries will put the emphasis on the UK to put its own house in order, rather than agree new collective steps, Norbert Röttgen, chairman of Germany’s parliamentary foreign affairs committee, said the case showed the need to target the fudged boundary between the Russian state security apparatus and organised crime. “The case should be a reason for Britain to examine its open stance towards Russian capital of dubious origin,” Röttgen said.

MEPs were questioning whether the UK would use new legislation – such as unexplained wealth orders that are included in the Criminal Finance Act 2017 – to challenge some Russian millionaires living in London. Ministers have insisted the orders will be used but say they cannot risk using them in a way that undermines the UK’s reputation for judicial independence.

Röttgen is seen as close to the German chancellor, Angela Merkel, who on Monday had expressed the hope that the re-election of Vladimir Putin as president this weekend would represent an opportunity for Europe to propose a fresh start with Russia over how to handle Ukraine and the annexation of Ukraine.

Discussing the EU’s approach to sanctions, one official said the Salisbury poisoning episode “makes it more likely we keep what we have got. The consensus has been beginning to crack”.

Hardly any frontrank politician in Italy, for instance, favours sanctions against Russia even if some oppose them simply because they are seen as ineffective.

Facebook Twitter Pinterest No EU country has so far committed itself to further sanctions against Russia if the UK declares the Russian state had acted unlawfully on British soil. Photograph: Jack Taylor/Getty Images

The EU has recently become more focused on the threat posed by Russia in the western Balkans as well as the continued occupation of parts of Georgia, where deaths occur most days. The criticism of Russian behaviour in the Balkans, especially by the US, raises hackles in parts of the region. On Monday the Serbian defence minister, Aleksandar Vulin, derided recent US warnings of Russian interference in the Balkans as some of the most dangerous statements made since the western military alliance bombed the country in 1999.

Inside Nato high command support for the UK position is strong, but the presence of Turkey, increasingly an ally of Russia in Syria, makes decisive action harder to coordinate.

The episode underlines the extent to which UK’s post-Brexit foreign and security relations with the EU remain unresolved. Britain as a third party would not be automatically consulted on common foreign and security policy, including further sanctions coordination, but both sides have been deferring talks until the outline of a trade deal becomes clearer.

The level of EU engagement with UK requests for help over Russia may influence the atmosphere of the Brexit talks. Speaking in London, Nathalie Tocci, special adviser to the EU head of external affairs, Federica Mogherini, said the scope for a win-win deal on foreign policy and security for both the UK and the EU was higher than in any other area, adding that the bargaining power was more balanced in this field than in any other.

Some EU officials have suggestedthat even in a transition period, when the UK is confined to the role of rule taker, it might be possible to construct a pilot in which the UK defence contribution remained integrated, and the country was fully consulted on decision-making.