Police have continued search for the item contaminated with the nerve agent that poisoned the couple. On Friday, investigators in protective clothing entered a John Baker House in Salisbury, where one of the victims, Dawn Sturgess, 44, lives.
Police believe Charlie Rowley, 45, and Ms Sturgess were exposed to Novichok after handling the unknown object.
The pair remains in a critical condition in hospital.
A government scientist who spoke with BBC News said the item was unlikely to have been left in the open before they touched it.
BBC home affairs correspondent Daniel Sandford also said the search for the item could take “weeks or months” and that no objects have yet been collected for testing.
Over time, rainwater and sunlight can weaken the effectiveness of Novichok , meaning the couple came in contact with the contaminated item in a contained space, the government scientist said.
However, other experts have said the nerve agent was designed to be persistent and not break down.
“Novichok is so toxic that it can pass through the skin and need not to be ingested” says the government scientist who spoke to BBC security correspondent Frank Gardner.
The source added that Mr. Rowley and Ms Sturgess’s symptoms were the exact shown by former Russian spy Sergei Skripal and his daughter Yulia.
They were both poisoned with the deadly nerve agent in nearby Salisbury in March.