Inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) were seen arriving at the Mill pub, where Mr Skripal and his daughter, Yulia, had a drink on the day they were poisoned.
The pair were later found unconscious on a bench outside The Maltings shopping centre.
They remain critically ill in hospital.
Tensions over the attack, which has been blamed on Russia, have escalated as Boris Johnson compared the upcoming World Cup in Moscow to the 1936 Olympic Games under Adolf Hitler.
The Foreign Secretary said it would be sickening to watch the Russian President, Vladimir Putin, host the World Cup in the summer.
Theresa May has already said no Royals or ministers will attend the World Cup.
The Russian foreign ministry's spokeswoman, Maria Zakharova, said Mr Johnson was "poisoned with venom of malice and hate, unprofessionalism and boorishness", adding that "it's scary to remember that this person represents the political leadership of a nuclear power".
She said on Facebook that Mr Johnson's comments reflected London's efforts to cast Russia as an enemy using the most absurd reasons in order to boycott the World Cup.
"But at what price?" she said. "At the price of provocations, setting nations and people against one another and undermining international peace and stability. Isn't the price too high?"
Ms Zakharova noted Mr Johnson's comments about the 1936 Olympics and the World Cup were an "unacceptable and unworthy" parallel towards Russia, a "nation that lost millions of lives in fighting Nazism".
Britain and Russia have expelled 23 of each other's diplomats over the attack, and Britain is seeking to rally allies for fresh sanctions against Moscow.