As a general matter, an undertaking, whether or not it has a substantial degree of market power, is free to decide with whom it will or will not do business. An undertaking might not wish to enter into a trading relationship with another party for a variety of legitimate commercial reasons, for example because it has objectively justified concerns about the creditworthiness of the other party.
However, a refusal to deal by an undertaking with a substantial degree of market power is likely to be abusive in limited circumstances.
For further information, please refer to Guideline on the Second Conduct Rule (in particular, paragraphs 5.16 to 5.22).