An executive sacked from one of the UK's biggest property companies began his defence today of an employment tribunal decision that he can claim he was unfairly dismissed because of his belief in climate change.

Tim Nicholson, 42, from Oxford, says his views on the environment are so strong that they led to clashes with other senior staff at Grainger plc.

In March, employment judge David Neath gave him permission to take the firm to tribunal over his treatment.

But the ruling is being challenged by Grainger on the grounds that green views are not the same as religious or philosophical beliefs.

Representing the company, John Bowers QC said: "A philosophical belief must be one based on a philosophy of life, not a scientific belief, not a political belief or opinion, not a lifestyle choice, not an environmental belief and not an assertion of disputed facts."

The firm claims that environmental concerns are based on scientific fact, whereas philosophy "seeks to answer the fundamental questions of human experience.

"Why are we here? What are the appropriate ethical values to apply to life? Issues on which there cannot be a scientific answer".

Mr Nicholson claims that his views on climate change affect his whole lifestyle.

He does not travel by plane and has renovated his home to be more eco-friendly.

In a witness statement to the previous hearing, he said: "I have a strongly held philosophical belief about climate change and the environment.

"I believe we must urgently cut carbon emissions to avoid catastrophic climate change.

"This affects how I live my life ... I encourage others to cut their carbon emissions and I fear for the future of the human race."

Mr Nicholson stopped working for Grainger as head of sustainability in July last year, having worked for the company since June 2006.

He claimed the company's chief executive Rupert Dickinson showed "contempt" for his concerns and once flew a member of staff to Ireland to deliver his Blackberry that he had left in London.

The judge will give his decision at a later date.