Dozens of Covid contracts had not been published when Boris Johnson told MPs they were "on the record for everyone to see".
The High Court has confirmed that 100 contracts had yet to be revealed at the time the PM made his comments.
Last month, a judge ruled the health secretary had acted unlawfully by not publishing contracts in the required timeframe.
Ministers said remaining contracts would be published as soon as possible.
The Department for Health and Social Care struck deals worth hundreds of millions of pounds during the coronavirus pandemic.
The government is required by law to publish a "contract award notice" within 30 days of the awarding any contracts for public goods or services worth more than £120,000.
In the High Court in February, Mr Justice Chamberlain said the public were entitled to see who the money was going to, what it was being spent on and how the relevant contracts were awarded.
He ruled that Health Secretary Matt Hancock acted unlawfully when his department failed to publish award notices for contracts it had agreed during the Covid pandemic within 30 days of them being signed.
The judge also found the health secretary had acted unlawfully by failing to comply with the government's own policy of publishing the contracts within 20 days of them being awarded.
The contracts have attracted scrutiny because many were awarded directly, without being opened to competition, because of the urgency of the pandemic.
Challenged about the ruling in the House of Commons on 22 February, Mr Johnson said: "All the details are on the record."
The prime minister added: "The contracts are there on the record for everybody to see."
But three days later, in a written legal response to the Good Law Project, seen by the BBC, government lawyers admitted 100 contracts for suppliers and services relating to Covid-19 signed before 7 October had yet to be published.
The court had asked the government to declare the number of contracts which had been published late.
Government lawyers also confirmed that 482 out of 513 contract award notices - 94% of them - had been published outside the 30 days required by law.
"It is only when details of contracts are published that they can be properly scrutinised, otherwise there is no way of knowing where taxpayers' money is going and why," says Gemma Abbott, the Good Law Project's legal director.
"We have a government contemptuous of transparency and apparently allergic to accountability."
The court order handed down on Friday by the High Court confirmed that 608 out of 708 relevant contracts had been published and that in some or all of these cases, the health secretary had acted unlawfully by not making them public in the required timeframe.
The Good Law Project said this showed the prime minister had "falsely reassured" MPs about the number of contracts that remained to be published.
The judge also ordered the government to pay £85,000 towards the campaign group's costs.
£34m mask contract
As officials attempt to the clear the backlog of unpublished contracts, more evidence is emerging of personal protective equipment (PPE) bought at vast expense that cannot be used as intended.
Last October the government published a contract summary detailing PPE worth £34m it had bought from the Chinese firm Qingzhou Yaowang Pharmaceutical.
The firm was contracted to supply high-grade FFP3 masks, which are used by medics working in intensive care units.
But the contract, which was only published this week, has revealed that officials knew the masks they had bought came only with ear loops.
Since signing the deal, the government has changed its technical requirements to say that these should not be used in masks of such a high grade because of concerns they do not fit securely enough.
Rachel Reeves MP, Labour's shadow chancellor of the Duchy of Lancaster, responding to reports that dozens of Covid contracts were yet to be published, said: "It is deeply concerning how much the government has relied on crony contracts throughout the pandemic, while leaving our frontline workers exposed.
"First the government acted unlawfully, then the prime minister falsely claimed everything had been published.
"It is a basic right for the public to know where and how our cash is being spent. The government needs to come out of hiding, simply publish the contracts it has sitting around and support our frontline staff with PPE not pay cuts."
A government spokesman said: "We have been working tirelessly to deliver what is needed to protect our health and social care staff throughout this pandemic, within very short timescales and against a backdrop of unparalleled global demand.
"This has often meant having to award contracts at speed to secure the vital supplies required to protect NHS workers and the public. We are committed to publishing all contracts and to date have published 99% of these in the Official Journal of the EU and we are working to publish outstanding contracts as soon as possible.
"As the 2020 NAO report recognised, all of the NHS providers audited were always able to get what they needed in time, thanks to the effort of government, the NHS, Armed Forces, civil servants and industry, who delivered over 8.8 billion items of PPE to the frontline at record speed."