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 This link will open in a new windowGuideline on the Second Conduct Rule (in particular, paragraphs 5.16 to 5.22).