The EFL's new Embargo Reporting Service - which has come into effect this month after clubs approved an amendment to the regulations to allow the governing body to publish a list of those clubs under registration embargoes on their official website - announced the development on Wednesday night
The Championship clubs who are now operating under restrictions are Hull, Derby County and Reading.
Hull’s embargo relates to a loan which it applied for and received from the governing body to help it through the coronavirus pandemic.
One of the prerequisites of receiving funds was for transfer business to be severely restricted.
A club statement read: "It is no secret that the effects of the Covid-19 pandemic have wreaked havoc across football, and unfortunately the impact upon Hull City as a business has been significant.
"To mitigate the financial pressures the club is facing, we have taken out a short-term interest free loan from the EFL/PL (Premier League) to enable us to keep the core of our squad together and avoid being forced into player sales at a time when the transfer market is unlikely to deliver strong results for selling clubs.
"Upon taking the loan, a number of conditions were imposed upon us and we were fully aware that we would be subject to certain limitations and restrictions in the transfer market until such time that the loan is repaid.
"These limitations include a squad size of no more than 25 established players, a cap on individual transfer fees and a limit on weekly wages.
"After such a successful season in League One, securing our first league title since 1966, our number one priority was keeping the group of players that delivered that achievement together.
"Taking out this loan will allow us to do that, with the associated restrictions causing little impact on our planned transfer activity.
"Since securing promotion, we have received a number of significant bids from Premier League clubs for our players, all of which have been turned down.
"Without the loan from the EFL/PL we would not have been in a position to do that and the only option would have been to sell players, perhaps at figures lower than our valuation.
"Whilst we are subject to restrictions around our transfer dealings, we are still able to bring players in and we are delighted with the quality and number of additions we have already made to the squad.
"Though we are in a position where we don’t need to sell players, if we receive bids that we think offer fair value, player sales would allow us to settle the loan and remove transfer restrictions."
Hull - operating under a modest Championship budget - brought in Brighton loanee Ryan Longman earlier this week, the club’s fifth arrival of the summer after the earlier captures of Nathan Baxter, Randell Williams, George Moncur and Andy Cannon.
Longman and Baxter have arrived on loan from top-flight clubs, who are expected to pay the salaries of both, while Williams, Moncur and Cannon have arrived in East Yorkshire on free transfers.