The union said the US-based firm - which remains the world's biggest maker of PCs - will axe 1,124 posts in total including 618 from its Bracknell hub, based in Cain Road.
Staff who work from home would also be affected, Unite said, adding that the company blamed falling demand and reorganisation for the job losses it wants to implement in the first three months of the year.
Unite national officer Ian Tonks said: "For the last five years HP has been addicted to a culture of job cuts in the UK to such an extent that its highly skilled workforce has little faith in the way the company is being managed and will be going forward.
"Unite will be doing everything possible to mitigate these job losses which are a hammer blow to the UK's IT sector and very distressing for employees in the run-up to Christmas."
Unite said 618 jobs could be lost at the Bracknell hub.
Vanessa Godsal, from Hewlett Packard, said: "HP commenced consultation for Q1 FY14 on November 28th, 2013 in the UK regarding potential workforce changes for 2014.
The proposed UK workforce management plan is part of HP’s global multi-year productivity initiative that was announced on May 23, 2012, and updated at its Securities Analysts Meeting on October 9, 2013, to address current market and business pressures in support of HP’s turnaround in EMEA.
HP remains committed to supporting the employability of its employees through a number of internal initiatives, including re-skilling, redeployment and support to obtain alternative employment as appropriate."