According to Sky News, which exposed the secrecy around the handling of the £75m contracts, the deals were never publicly announced, and were not published on the government website until eight months after they had started.
The government awarded nine consultancy firms, including Deloitte, EY and PwC, contracts worth between £5m and £10m, to supply the Cabinet Office with “consultancy support for EU Exit”.
These contracts will run until end of April, with the option to have them extended.
The £75m is the maximum budget set aside for this kind of contracts, meaning the actual amount spent may be considerably less.
It is thought they cover programme and project management, operational delivery and commercial expertise.
Meg Hillier, chair of the Public Accounts Committee, told Sky News that the secrecy around the contracts was “ridiculous” and revealed she would be asking the National Audit Office (NAO) to investigate them.
She told Sky News, “We've talked to them before about other contracts and they have the ability to go into a contract and see, in details, what's involved.
"They will have to make a judgement about what is possible to reveal - but really the government should just bring this out itself.
"This is ridiculous. We're within three months of us leaving the EU as it stands. We should know what the government is spending and what it's spending it on. Let's not forget that this is taxpayers' money."
Joe Owen, from the Institute of Government, also told the publication that there has been lack of transparency regarding Brexit preparations.
He said, "There's definitely been an increase in secrecy more generally across the civil service as a result of Brexit, just because of how politically difficult it's been for many reasons.
A government spokesperson said, "It is standard for government departments to draw on the advice of external specialists to deliver key government policy. All government contracts are published online - and these are no exception."
PwC, Deloitte and EY have declined to comment. The NAO said it has not received Hillier's request yet.
It was unveiled in August last year that the government would be paying Deloitte £2.6m to train its staff in post-Brexit international commercial disputes.
The contract followed a clash between the Big Four firm and the government in 2016, when prime minister Theresa May’s office accused Deloitte of “touting for business” through an “unsolicited” analysis of the government’s Brexit strategy.
The memo was highly critical of May's leadership on Brexit, claiming she was “drawing in decisions and details to settle matters herself”. It said the prime minister’s attitude was unsustainable and could lead to senior civil servants having to intervene.
Deloitte said at the time that it regretted the publication of the note, and apologised for the “unintended disruption it caused government".