Staff security concerns have been "misjudged"
Since the beginning of last week up to 2,000 staff have been on an indefinite strike at more than 50 offices across the UK.
The Public and Commercial Services Union (PCS) is currently balloting its 70,000 members on an all-out strike, which could start in December, over the removal of security screening in offices.
On Wednesday PCS leaders were in London to discuss increasing security fears of workers in agency officers.
The attacker is still being hunted by police.
After the office re-opened on Tuesday staff refused to continue working.
Removal of screens has begun in a number of centres around the country, as part of the government's decision to merge job centres and benefits offices into one service.
Jim Hanson, of the PCS, said the ballot will be closed on 23 November, and announcements on the strike made a few days later.
He told BBC News Online: "Management have misjudged the level of concern among our members on this issue.
'Adverse reaction'
"With the merging of the two services, these offices have become a last resort place for people who are applying for benefit.
"If those people are refused these they have nowhere else to go and staff are very concerned they will face adverse reactions about this."
Mr Hanson said staff regularly face verbal abuse from clients, and this can sometimes erupt into violence.
"For example, the office in Welwyn Garden City was smashed up recently, which shows this problem is not confined to the big cities," he said.
"And in Croydon in August, a policeman was stabbed in one of the offices.
"There was no proper consultation on this issue before it was implemented."
Security improvements
However, a spokesman for the Department of Work and Pensions agreed that staff should be able to work without fear of attack.
He said: "These new offices will actually be safer than those in the old job centres and benefits agencies.
"We feel that the removal of the screens is an important part of developing a new service for clients where they can talk about jobs and benefits in one place.
"Because this is done on a one-to-one basis, we do not feel it will work with a screen separating staff from the client."
He said security was being stepped up in all offices.