The Pensions Regulator has escalated its investigation against Sir Philip Green over the collapse of BHS.
The watchdog said in a statement it has begun enforcement action "to seek redress on behalf of the BHS pension schemes".
Warning notices have been sent to the retail tycoon and his holding company Taveta, as well as serial bankrupt Dominic Chappell and his firm Retail Acquisitions Limited, to whom Sir Philip sold the business for £1.
The move could see see the former BHS owner being ordered to pay towards the company's pensions deficit.
The high-street chain collapsed in April with the loss of 11,000 jobs and a £571m pensions deficit shortly after being sold for £1 by Sir Philip to Mr Chappell.
Pensions Regulator chief executive Lesley Titcomb said: "We have been clear in our public commitment to make significant progress by the end of 2016 and the issue of these notices meets that commitment.
"Our decision to launch enforcement action is an important milestone in our work to attain redress for the thousands of members of BHS schemes who have been placed in this position through no fault of their own.
"Issuing warning notices at this time reflects the outcome of our investigations and that we are yet to receive a sufficiently credible and comprehensive offer in respect of the BHS schemes.
"We continue to pursue the best deal for members of the BHS pension schemes. If parties wish to approach us with settlement offers, that course remains open to them."
The notices sets out the arguments and evidence as to why the watchdog thinks Sir Philip and Mr Chappell should be liable to support the BHS pension schemes.
Sir Philip and Mr Chappell will now have a specified period of time to respond to the notices.
Commons Work and Pensions Committee chair Frank Field said: "We are not surprised that the Pensions Regulator has, like all the rest of us, lost patience with Sir Philip Green's excuses and empty promises.
"It seems clear Sir Philip would rather kick the can down the road and avoid responsibility than come up with any fair, sustainable settlement for BHS pensioners."
This latest move comes after MPs unanimously recommended Sir Philip be stripped of his knighthood.
They asked the Honours Forfeiture Committee to ensure Sir Philip's knighthood is "cancelled and annulled", with the move viewed by one former minister as part of the businessman's "humiliation".
Last month Sir Philip Green told ITV News in an exclusive interview he is "sad and very sorry" for the hardship caused by the collapse of BHS.
Sir Philip Green said in a statement: “I have read the statement from the Pensions Regulator this evening and noted its contents.
"I have provided the Regulator with what I believe to be a credible and substantial proposal, with evidence and bank confirmation of cash availability, which would prevent the scheme from entering the Pension Protection Fund.
"This is in order to achieve a better outcome for the BHS pensioners.
“I have also spoken to the Chairman of the Trustees who is supportive of the proposal on the basis that it provides members with better benefits than they would receive from the PPF.