Shelter employs 850 staff and helps 170,000 people annually in the UK

Workers voted by 211 to 78 for industrial action after the charity's management extended working hours in order to plug a gap in funding.

Film director Ken Loach has backed staff, saying their cause is justified and urging a boycott of the charity.

Shelter's Chief Executive Adam Sampson called his comments "unfortunate".

Bitter dispute

Mr Sampson said: "Mr Loach is in effect calling for the end of Shelter as an organisation after it has spent more than 40 years helping millions of people in housing need."

The charity says the contractual changes will help it fulfil its current obligations to the public sector and meet a funding shortfall partly caused by the worsening economic climate.

Under the new contract, staff will have to work an extra two and a half hours a week for no extra money.

Their right to an annual incremental pay rise is being scrapped and, under the new contract, salaries will only increase in line with inflation.

Salary increases alone cost it over �1m last year.

Changes 'outrageous'

But the union says their rights are being destroyed.

"In recent times, Shelter has spent at least half a million pounds on refurbishing its head office", said Unite's regional industrial organiser, Alan Scott.

"Our members believe that some of this money could have been used to protect their agreed terms and conditions of employment."

When the strike action was announced last week, Ken Roach told BBC Radio 4's Today programme the changes were outrageous.

"These are people who work with great dedication", he said.