The “Occupy Westfield” campaigners have set up camp on the stalled site to raise awareness of delays on the job.
Work first started in 2004 but stopped in 2008 leaving an empty site in the city centre.
Campaigners are refusing to budge until a construction start date is agreed.
Westfield won revised planning permission for a smaller £275m scheme in October but little progress has been made on site since.
Protesters entered the site over the weekend and Westfiled chiefs have now written to them asking them to leave.
Westfield told the campaigners: “We appreciate your concerns but would like to reassure you that Westfield remains committed to progressing the scheme and is currently undergoing detailed negotiations to secure retailers as well as other important predevelopment activities which by their nature are not physically evident on site.
“Westfield has safety concerns as you are currently trespassing on a construction site. By entering and occupying the site, you are not only endangering yourselves but also others.
“The site needs to be properly secured and managed to avoid the very real possibility of injury both to you and the wider Bradford public. We urge you to leave the site immediately so that we can secure it and remove the risk of accidents.”
The campaigners said they would not leave without a firm date for construction to start.