Along with Man City and Everton, Chelsea are now subject of an investigation by the Premier League for alleged financial breaches.
That is according to the Times, who report suspicions over payments around two signings from Russian club Anzhi Makhachkala in the summer of 2013.
Anzhi were, at the time, attracting headlines for their high-profile acquisitions, but the Dagestani outfit dissolved less than a decade later in 2022 due to money issues.
Their record sale came with Willians 30 million move to Chelsea, with the Brazilian winger joined at Stamford Bridge the following day by Samuel Etoo.
Chelsea famously beat Liverpool and Tottenham to a surprise deal for Willian, which is now alleged to be due to payments made to Russian entities.
The Times Martyn Ziegler and Matt Lawton report that secret payments were made separate to those to Anzhi, under the previous ownership of Roman Abramovich.
A free transfer taking Etoo to Chelsea from Anzhi also involved an unsolicited fee, while there are further allegations of payments to offshore companies and a players family.
In a statement, Chelsea have acknowledged potentially incomplete financial reporting, which was uncovered during due diligence upon the 2022 takeover.
The report continues: The alleged payments, which run into seven figures, appear not to have been registered as part of the clubs annual financial reporting to the FA, Premier League and UEFA, which would be in breach of their rules.
Despite the change in ownership, with the club bought last year by a consortium involving American businessman Todd Boehly, it is explained that Chelsea could still face sanctions.
Those could include heavy fines and points deductions, though it is explained that the club self-reporting the issues may be used as mitigation.
Earlier this year, Chelsea reached a settlement with UEFA relating to financial breaches in the Abramovich era, and according to the Times it seems certain this will influence any charges from the Premier League.
Willian spent seven years at Stamford Bridge, playing 339 times, with spells at Arsenal, Corinthians and currently Fulham to follow.
Meanwhile, Etoo was with Chelsea for a single season, and after being considered for a move to Liverpool in 2014, he went on to join Everton on a free transfer instead.