The London Bridge terrorists started preparing their atrocity more than two weeks before they struck, while the ringleader Khuram Butt was under MI5 investigation, an inquest has heard.

Three 12-inch knives used in the attack on 3 June 2017 were bought 19 days earlier from a Lidl supermarket in east London. During the attack a rented van was used to run over people on London Bridge, killing two, before three terrorists sprinted out to stab to death six people.

The inquest heard new details of the attack and preparations, including that Oxford Street in central London may have been the original target. It also heard that a bungle by the terrorists meant they missed out on hiring a large seven-and-a-half-tonne lorry, which almost certainly would have caused more deaths and inquires.

Butt, 27, carried out the attack on London Bridge with Rachid Redouane, 30, and Youssef Zaghba, 22.

CCTV played at the inquest showed Redouane buying the distinctive pink knives on 15 May 2017 after entering Lidl in East Ham at 10.15pm.

The inquest saw footage showing the attackers meeting in the days before the attack, in and around the Ummah fitness centre in east London.

Acting DCI Wayne Jolley, from Scotland Yard’s counter-terrorism command, told the inquest a hire van rented from Hertz used in the attack cost the terrorists just £70. They had wanted to use a large lorry, similar to the attack in Nice in 2016, but made an error after reserving it from a hire company and never got hold of it.

Jolley said: “It was never collected. It is our assessment that the branch he wished to collect it from had closed at 11am that morning.”

They spent the day of the attack seeing family and by early evening picked up the van and went to a B&Q DIY store in Harold Hill, east London, to buy 29 bags of gravel to give the van extra weight.

The three attackers, who lived in east London, drove the van towards London Bridge, first arriving at 9.58pm, some eight minutes before the attack, and driving along the bridge before performing a U-turn and heading back over the bridge.

Jonathan Hough QC, counsel to the inquest, suggested it was possible the terrorists decided on London Bridge as a target “en route”. He revealed that an area in central London thronging with people on a Saturday evening in summer may have been the original target.

Hough asked: “Did they have a phone in the van with directions to Oxford Street showing on it?”

“Yes they did,” Jolley replied.

After the attack ended with the three terrorists being shot dead, petrol bombs contained in 13 bottles of rosé wine were found in the van’s footwell, with a cloth wick and two blow torches to light them also recovered. The bottles for the “incendiary devices” were bought from the same Lidl as the knives on 30 May, four days before the attack.

MI5 began investigating Butt for terrorist attack-planning in 2015 but had found no evidence. He was still a target of investigation by June 2017, but mainly for possible travel overseas, a government commissioned report suggested.

The eight people who died in the attack, along with the perpetrators, were Sara Zelenak, 21, Alexandre Pigeard, 26, Christine Archibald, 30, James McMullan, 32, Kirsty Boden, 28, Sebastien Belanger, 36, Ignacio Echeverria, 39, and Xavier Thomas, 45.

The inquest continues.