© Rex/Shutterstock Yulia Skripal is said to be no longer in a critical condition.

The Russian foreign ministry said it would be announcing retaliatory measures shortly.

Zakharova said Britain’s behaviour ran counter to a 1968 consular agreement signed between the then Soviet Union and Britain under which Moscow was meant to have access to its nationals on British soil and to be able to give them advice.

Zakharova said nobody had cancelled the agreement, which she said still had force in international law. The UK is likely to argue that an agreement between the UK and the Soviet Union is not enforceable in court, and there is no reason to give the Russians access to a woman that it claims was responsible for her attempted murder.

Yulia Skripal is said to be no longer in a critical condition.Photograph: Rex/ShutterstockYulia Skripal was said to be “improving rapidly” and no longer in a critical condition. Her father remains in a critical but stable condition, Salisbury NHS Trust said on Thursday.

Russia also challenged claims made by Dean Haydon, Britain’s counter-terror police chief, that the Skripals first came into contact with the nerve agent from their front door.

“Traces of the nerve agent have been found at some of the other scenes detectives have been working at over the past few weeks, but at lower concentrations to that found at the home address,” Haydon said.

Investigators had sealed off the bench where the Skripals were found, a pub and restaurant the pair visited, and the grave of the former spy’s wife.

Maj-Gen Alexander Mikhailov, from the Russian security agency FSB, claimed May had “screwed up” and if it was true the poison was found on the doorstep then the Skripals would have died instantly and not made it as far as a park where they were found slumped on a bench.

He said May should resign because she misled world opinion.

Both sides are waiting for a report back from the Organisation for the Prohibiton of Chemical Weapons, which sent experts to visit the scene in Salisbury and is studying samples of the nerve agent.

Dmitry Peskov, the press secretary of the Russian president, Vladimir Putin, responded to reports in the Guardian that May is looking at whether she can take steps to restrict London city institutions from handling Russian sovereign debt.

The proposal has been pressed upon her by the chairman of the foreign affairs select committee, Tom Tugenhadt, and Russian exiles living in London.

Related: May considers banning City of London from selling Russian debt

Peskov said it was difficult to guess what other options for anti-Russian measures Britain can take.

“As for the UK, due to the fact that now it is a fairly unpredictable country in relations with the Russian Federation, it is difficult for us to judge what other options can be considered, and what can be the basis for this,” he said.

In Germany, the government’s decision to expel Russian diplomats this week in solidarity with the UK has widened splits, with more senior politicians demanding the UK comes forward with concrete proof of Russian involvement in the poisoning.

The deputy leader of the Social Democrats, Ralf Stegner, criticised the expulsion in an interview for the Süddeutsche Zeitung newspaper, saying he feared an escalatotion in measures “which can still hurt us very much”. He said “appearances and plausibility were not enough to convict”.