Had the Pakistani-origin London Bridge attacker succeeded in hiring a 7.5 tonne truck, the number of casualties in the carnage would have been much higher, the British media reported on Saturday.
Ringleader Khuram Shazad Butt's efforts to hire the bigger truck failed when his payment was declined, the BBC reported.
Butt, 27, had ganged up with two other accomplices to carry out the terror attack on June 3 that killed eight persons and injured nearly 50.
Commander Dean Haydon of Scotland Yard's counter-terrorism command said: "Concerningly, Butt had earlier attempted to hire a 7.5-tonne lorry that same morning. When he did not provide payment details, the rental did not go ahead. The effects could have been even worse".
The truck they had wanted to hire was smaller but similar to the one used in last year's Nice attack in which more than 80 people were killed in France.
The aim of the attackers seems to have been to replicate the attack in Nice, the officer said.
The three terrorists had tied 12 inch (30cm) pink ceramic knives to their wrists and had petrol bombs in the van.
Police discovered 13 wine bottles containing flammable liquid with rags stuffed in them, essentially Molotov cocktails, and blow torches to ignite them, potentially meant for another wave of attacks.
Butt drove the van into pedestrians on the iconic bridge. He then went on a stabbing spree along with Rachid Redouane, 30, of Moroccan and Libyan descent and Moroccan-Italian Youssef Zaghba, 22, in Borough Market.
The trio were shot dead by the police.
The Metropolitan Police said the men had rented a flat in Barking to use it as a safe house.
Police have discovered equipment for making petrol bombs, plastic bottles and duct tape for constructing fake suicide bomb belts, and an ID card for Redouane.
"We also need to watch out for those who may be inspired by these recent attacks," said Haydon.
Butt hired the van used in the attack from a B&Q store in Romford in the morning of the attack, it said.
On June 3, the van crossed London Bridge heading south. Six minutes later it returned, crossing over the bridge again and making a U-turn at the northern end, the report said.
The attack began with the van driving back along the pavements of the bridge, running down three pedestrians before crashing outside the Barrowboy and Banker pub opposite London Bridge station, it said.
They stabbed five people who had been enjoying the area's pubs and restaurants.
In the back of the white Renault van used in the attack officers found wine bottles filled with a flammable liquid with rags tied to their necks, the report said.
There were blow torches for lighting these "Molotov cocktails". They had added bags of building gravel and several chairs possibly to suggest a reason for having hired the van.
The massive police probe into the attack continues and by Saturday, 20 arrests have been made and 13 buildings searched.
Meanwhile, police said they have arrested two men linked to the attack on suspicion of preparing terrorist acts after carrying out raids in Ilford and Barking, London.
A total of seven persons now remain in custody in relation to the probe into the attack.
Scotland Yard, seeking more witnesses to come forward, is appealing to companies who hire out vans to report any suspicious activities.
So far, 282 witnesses from 19 countries have been questioned but police still want more people who saw what happened to come forward.
Police are also appealing for information about the "distinctive" pink "Ernesto" brand knives the men were carrying. They said they had not found evidence of anyone else being involved in the plot - or inspiring the attackers to carry it out.