Indeed, the organisation is disappointed that several large businesses are "undermining the spirit of the PPC" by increasing payment times for suppliers.
Small businesses are among those to be affected by late payments as it reduces their cash flow at a time when they need it most.
Chief Executive of the Forum, Phil Orford, said that many firms are anything but ethical to their suppliers, despite signing up to the code.
"We think that standard net monthly, that is payment on or by the end of the month following the month of invoice, is more than ample time to settle up," he added.
"Our own research last year showed in fact most local councils are paying in under 30 days, with many in under ten days, so if notoriously inefficient local governments can cough up this fast, we see no reason why big business can't."
It comes after the government vowed last month to tackle late payments to help small firms.
Posted by Thomas Fletcher