A trade union has asked how big the fall in the number of employment claims needs to be before the imposition of tribunal fees can be considered unlawful.

Unison opened a two-day hearing at the Court of Appeal this morning to argue against the government’s decision to bring in fees of up to £1,200 in July 2013. Two previous challenges failed after the High Court said there was insufficient evidence to link fees with a denial of meritorious claims.

Representing Unison, Karon Monaghan QC of Matrix Chambers told the court that year-on-year comparisons showed significant drops in the number of new claims. The number of equal pay claims fell 83% after the introduction of fees, while sex discrimination claims fell up to 91% in the first quarter of 2014.

These falls, she argued, could not be caused by anything other than the fees – and this in itself was a contravention of European Union law.

‘Absent of any other explanation, it is my submission that on any rational inference these fees are imposing too heavy a burden,’ she said. ‘How big a drop would be required before one can infer these fees are imposing too heavy a burden?’

Questioned by Lord Justice Moore-Bick on whether the new fees were a ‘matter of priorities’, Monaghan said potential clients often had no choice but to drop valid claims – even if remission was available for those on a sufficiently low household income.

She added: ‘The drop is so dramatic the court is entitled to infer this is not a case of a woman choosing to have a haircut rather than bring a case. There will not be the resources to make that choice and yet they won’t be entitled to sufficient remedy to afford the balance of the fee.’

The MoJ said it will argue ‘strongly’ that its decision was lawful. That argument is set to be made on Wednesday once Unison’s submissions are complete.