Paul Quinn made 27 league and cup appearances for Cardiff last season
Motherwell have demanded immediate payment of a "substantial debt" owed by Cardiff City for the transfer of defender Paul Quinn.
Quinn joined the Bluebirds last summer for a fee in the region of £300,000.
Well say the money owed has been due since January, which would suggest payment of the fee was staggered.
"The board have been very patient with Cardiff given the financial pressures they have faced," said chief executive Leeann Dempster.
"However the interests of our shareholders, employees and fans are being jeopardised by the failure of Cardiff City to pay the debt that they have outstanding.
"We have made clear to the board of Cardiff that we are left with no option but to pursue every possible avenue of recourse to secure our own club's interests. We hope that this matter can be resolved in an orderly manner."
Cardiff City chief executive Gethin Jenkins said: "We are fully aware of the debt owed to Motherwell and we are working to resolve the issue as soon as possible."
It is the latest blow for the Bluebirds, who are under a transfer embargo imposed by the Football League because of continuing concerns over unpaid tax bills.
The club face a winding-up order at the High Court on 11 August over an unpaid PAYE tax bill owed to Her Majesty's Revenue and Customs which stands at £1.3m.
Cardiff released a statement earlier this month saying they had paid off one month of the bill but did not indicate whether the winding up order is still in place.
However, the transfer embargo still remains as the club has yet to lodge year-end accounts for May 2009.
Cardiff received a £6m investment in May from a Malaysian consortium headed by Dato Chan Tien Ghee, who succeeded Peter Ridsdale as chairman.
The club has estimated debts of £30m and faced their fifth winding-up order to settle a £1.9m tax bill on 16 June, before it was eventually withdrawn after the debt was paid.
Around half of that debt is owed to Langston Corporation whose representative is Sam Hammam, the former Cardiff City owner.
Controversial businessman Hammam brought an end to his six-year tenure as Cardiff City chairman in October 2006 after agreeing to sell his majority shareholding, with former Leeds United chief Ridsdale taking over the role at the south Wales club.
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