A leading animal welfare charity is locked in an acrimonious legal dispute with a former employee who claims he was dismissed for telling colleagues that its pension fund invested in companies that have been involved in animal testing.

The ex-employee, a vegan, describes himself as a whistleblower whose case will be the first to decide whether his beliefs should be protected under the Equality Act.

But the League Against Cruel Sports, which was instrumental in securing the fox-hunting ban, has rejected Jordi Casamitjana’s claims that he was dismissed for the reasons he has given and has pledged to defend itself.

Casamitjana, who is crowdfunding to raise money for his legal action, has instructed the leading law firm Bindmans to fight his case which will draw attention to the practice of “ethical veganism”. Unlike “health vegans” who choose not to eat meat, fish and dairy for their own personal wellbeing, ethical vegans consider the broader impact of how they spend their money on animal welfare.

On his crowdfunding website, Casamitjana says that this belief has “informed my daily existence, including my career and employment”.

It was only after checking the small print on the league’s pension, he says, that he discovered it was apparently “investing in companies, including pharmaceutical and tobacco companies, who are known to engage in animal testing”.

He had previously worked for the league from 2004 to 2007 and is understood to have had no problems with its pension fund investments then. But the fund changed its investment strategy three years ago. On the website Casamitjana, who returned to the league in 2016 as head of policy and research, said: “When I wrote to my colleagues to tell them that their pension was being invested in non-ethical funds, and that there were alternatives to the single alternative that the company was suggesting, I was sacked. I was given no appeal hearing.” Casamitjana was dismissed in April.

The league explained to the Observer that, until 2015, its staff were automatically enrolled into an “ethical” pension fund, but the Financial Conduct Authority, the body responsible for pensions, changed the regulations and the fund became ineligible.

Given the nature of our work, the League is often the subject of attacks from all sorts of quarters Andrew Knott, interim CEO

“We followed financial advice and changed our pension, while still offering the chance for individuals to change their own fund to an ethical fund,” said Andrew Knott, the league’s interim chief executive. “Mr Casamitjana was not dismissed because he raised concerns about the pension, either internally or externally, so there is no substance to his claims that he was ‘whistleblowing’.

“All staff were informed about the situation and the processes in place to enable them to make their own choices. The reason for his dismissal is different from that which he states but clearly this is confidential information which we are respecting.”

The case comes at a difficult time for the league. Its chief executive has stood down for health reasons and its chair is due to depart soon.

“Given the nature of our work, the league is often the subject of attacks from all sorts of quarters,” Knott said. “As a charity, we hold the views of our colleagues and supporters dear. While our main focus is on preventing cruel animal sports, we of course have many vegans in our workplace. We share vegan practices across our wider staff and when meeting with supporters, many of whom are not vegan. We reject any suggestions otherwise.”

When approached for comment, Casamitjana said: “Regrettably, I have been forced into a position where I am required to litigate to protect my rights as an employee and an ethical vegan. However I have no desire to cause any unnecessary difficulty for the charity or the wider animal protection movement. As a result . I will not be making a public statement at this time, save for the statement I have had to make in order to crowdfund my legal fees.”