The two men who pulled off Britain's biggest jewellery heist planned to carry out the robbery two days earlier, according to Scotland Yard.
CCTV images of the men wanted in connection with the £40 million raid at Graff Jewellers on Thursday August 6 were released showing them outside the store on Tuesday 4 August.
Det Ch Insp Pam Mace, head of Barnes Flying Squad, said: "We believe this was more than just a practice run, and that they were planning on committing the robbery on 4 August."
The CCTV images show the men outside the New Bond Street store at around 5.40pm on Tuesday and a blue BMW they later used as a getaway vehicle.
Ms Mace said: "The stills clearly show the men outside Graff Jewellers, looking into the shop, and the blue BMW driving through New Bond Street.
"There has been a £1 million reward offered in connection with the identification, arrest and prosecution of the persons responsible, and the recovery of the stolen property offered up by Tyler and Company on behalf of the interested insurers, and I am urging anyone who has information about those responsible, or where the jewellery is to come forward.
"They are wearing similar clothing to when they committed the offence on Thursday August 6, however the white man is wearing, in these stills, a purple shirt and no tie.
"What happened on Tuesday August 4 after they left the area? Did you see them or the BMW? We need you to come forward and speak to us please."
A Scotland Yard spokesman said there was no indication why the men did not carry out the raid on Tuesday.
The robbers fired a gun during their escape on August 6 and again as they got into a silver Mercedes in nearby Dover Street and were challenged by drinkers at a pub.
Police have already released details of the getaway cars and said the robbers used cloned number plates.
The men abandoned the BMW in a nearby street after it hit a black taxi just seconds after the raid.
Officers also found a high-powered motorbike possibly used to spirit away a bag holding the stolen gems.
Scotland Yard described the first vehicle as a silver B-class Mercedes with the registration number LV06 HFA.
The car drove on into Farm Street where it is believed some of the suspects got into another car.
The second vehicle was described as a blue Volkswagen Sharan with the registration number RA07 XEV.
A police spokesman said the registration plates were known to be cloned.
Officers arrested and questioned a 50-year-old man in Ilford, east London, on Monday in connection with the robbery.
They suspect the man, who has been bailed, may have helped plan the raid although he was not in the West End when it took place.
The incident room number is 020 8247 4804, or call Crimestoppers anonymously on 0800 555 111.Reuse content