According to ITV, STV has failed to honour its "contractual contributions" towards the overall Network Programme Budget, thus leaving an extensive unpaid debt. ITV also indicated that it has been trying to resolve the situation for over a year.
As an ITV licence holder in central and north Scotland, STV can select whichever programming it wants from the ITV network.
However, it has recently shunned a series of high-profile ITV drama on its peak-time schedule in order to prioritise more Scotland-focused programming.
It also recently revealed plans to stop broadcasting the ITV1 standalone 6.30pm national news bulletin in favour of a news programme offering a "Scottish perspective" on national and worldwide stories.
"STV is attempting retrospectively to opt out of an increasing number of peak-time programmes which contravenes existing agreements," said ITV in an official statement.
"The company is also wrongly attempting to claim a rebate against programmes which have been "written off". We are currently withholding monies from STV against this debt and currently believe the net debt is approximately £15 million to £20 million.
"We have been attempting to resolve this matter for more than a year but unfortunately our efforts have been unsuccessful. Given that we are a commercial organisation, with responsibilities to our shareholders, we are left with no option but to take legal action to recover this sizeable debt."