Detectives are trying to pin down the movements of the BMW owned by ex-spy Sergei Skripal on the morning before he was found critically ill.
Scotland Yard has released a photo of the burgundy BMW 320D saloon car - registration HD09 WAO - which has become the focus of the huge investigation into the nerve agent attack.
Mr Skripal, 66, and his daughter Yulia, 33, were found slumped on a bench in Salisbury city centre, in Wiltshire, on March 4.
Counter-terrorism police believe Mr Skripal's car may have been seen in the areas of London Road, Churchill Way North and Wilton Road at around 9.15am.
Salisbury cemetery and crematorium, where Mr Skripal's wife's grave and a memorial to his son are located, is situated on London Road and has been cordoned off by police who were seen removing items earlier this week.
The car was then later seen being driven down Devizes Road, towards the town centre, at around 1.30pm.
Metropolitan Police Assistant Commissioner Neil Basu said: "We are learning more about Sergei and Yulia's movements but we need to be clearer around their exact movements on the morning of the incident.
"We need to establish Sergei and Yulia's movements during the morning, before they headed to the town centre. Did you see this car, or what you believe was this car, on the day of the incident?
We are particularly keen to hear from you if you saw the car before 1.30pm. If you have information, please call the police on 101."
Miss Skripal arrived at Heathrow from Russia at 2.40pm on March 3 and the pair visited Salisbury city centre at around 1.40pm the following day, going to the Mill pub before eating at Zizzi restaurant between 2.20pm and 3.35pm.
They were discovered on the bench in The Maltings shortly after 4.15pm, when police were called.
Mr Basu said the probe, which is being worked on "around the clock" by around 250 officers, could take "months".
He said: "In any investigation, the information we receive from the public can be crucial to helping the police build a picture of events and in this case the public response has been immense.
"Around 400 witnesses have already given statements, with hundreds more to be taken in the coming days.
"To date, detectives have recovered 762 exhibits and are trawling through around 4,000 hours of CCTV. They are making good progress in what is a painstaking investigation that is likely to be ongoing for weeks, if not months."
Anyone who has images or footage that may assist the investigation is asked to upload them at the secure website http://www.ukpoliceimageappeal.co.uk.