A man who claims he was unfairly dismissed from his job because he believes in climate change is attempting to have his environmental views recognised under religious law.

Tim Nicholson, 42, says his beliefs on the environment are so strong they led to clashes with other senior staff at Grainger, one of the UK's biggest property companies.

He said the chief executive, Rupert Dickinson, showed contempt for his concerns and once flew a member of staff to Ireland to deliver his BlackBerry, which he had left in London.

In March, employment judge David Neath gave Nicholson permission to take the firm to a tribunal over his treatment. The company is challenging the ruling, arguing that environmental beliefs are not the same as religious or philosophical ones.

Nicholson, from Oxford, said his views – which compelled him to make his home more eco-friendly and do not allow him to fly – affect his entire life. 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."

He stopped working for Grainger as head of sustainability in July last year, having been at the company since June 2006. At an employment appeal tribunal in central London today, Dinah Rose QC, for Nicholson, said: "The philosophical belief in this case is that mankind is headed towards catastrophic climate change and that, as a result, we are under a duty to do all that we can to live our lives so as to mitigate or avoid that catastrophe for future generations.

"We say that that involves a philosophical and ethical position. It addresses the question, what are the duties that we own to the environment and why?"

She told Mr Justice Michael Burton – who ruled last year that Al Gore's environmental documentary An Inconvenient Truth was political and partisan – that beliefs about "anthropogenic climate change" could be considered a philosophy under the Employment Equality (Religion and Belief) Regulations 2003.

John Bowers QC, representing Grainger, said Nicholson's views were based on scientific fact and were predominantly political. "We would say that because it is political, it is dealing with an assertion of fact," he said. "It is a scientific view rather than a philosophical one. Philosophy deals with matters that are not capable of scientific proof."

A philosophy must have a comprehensive belief system. "What Mr Nicholson asserts is a scientific claim that if we don't urgently cut carbon emissions, we will not avoid catastrophic climate change. There is nothing philosophical about that."

The judge raised the removal by Lady Scotland, the attorney general, from regulations that defined beliefs as religious beliefs or those that were similar to religious beliefs of the word "similar" after atheist and humanist groups objected to comparisons of their philosophies with religion.

The result of the tribunal will determine whether Nicholson can pursue his claim for unfair dismissal.

The hearing continues.