The Green Party leader has backed a farmer's campaign against a proposed innovation centre being built on land his family have farmed since 1902.
The landowner wants to convert Milner Field Farm, Bingley, from agricultural use to outdoor scientific research.
Natalie Bennett said the plans would destroy a "successful" small business, but the landowner said the plan would create valuable high-tech jobs.
The land was once owned by the Victorian industrialist Sir Titus Salt.
David Downs, the current tenant, said: "We are the last dairy farm in Bingley now and we have been overwhelmed by the support we've got.
"It is not just about us it is the wider picture - this is the last piece of green space in the area."
The farm was once the parkland setting for the now demolished Victorian mansion of Milnerfield built by Sir Titus's son.
Sir Titus Salt
- Born in 1803 near Leeds, he began his career working as a woolstapler with his father Daniel
- He went on to buy his own cloth mill in Bradford and prospered
- Aware of the poverty and living conditions of his workers he built a new mill on the outskirts of Bradford in 1853
- Salt went on to create a whole village, Saltaire, for his workers, complete with church, school and a park
- Created a baronet by Queen Victoria in 1869 he died in 1876
- Saltaire was designated a World Heritage Site by Unesco in 2001
Ms Bennett, said: "It is part of a whole much broader issue of protecting the green belt, protecting the green spaces that are the lungs of so many communities.
"Also this is a successful small business, one that is producing food for the local area, and we need to protect our dairy farms."
In a statement, the landowner, Kingsbridge Directors Pension Scheme, said the planning application would not affect the "green belt" status of the land.
"The change of use of the fields from agriculture to outdoor scientific research (expressly not involving the erection of any additional buildings on those fields) maintains exactly the shapes, topography, trees, and views of the current fields."
It said the innovation centre would put Bradford at the "centre of the Northern Powerhouse initiative."
Bradford Council has not yet set a date to consider the application.