that is a hardware fault with the print head. Is it under a year old? If so, contact HP http://welcome.hp.com/country/uk/en/contact/contact_us.html
. They will arrange a swap out. If you are from the UK, then UPS will send out a new unit. You then pack your old unit in the box that the new one arrived in and call UPS to arrange free collection of the old one.
The whole procedure is free and they will drop off/pick up from a home or work address.
Hope this helps.