Vince Cable has privately clashed with the Tory justice secretary, Chris Grayling, over an alarming drop in the number of sexual discrimination cases being heard at employment tribunals since the government attached a fee to seeking legal redress.

The Liberal Democrat business secretary has ordered his officials to investigate into whether the fees are proving to be a barrier to justice.

The move came after repeated private requests for a review, including ones made at the cabinet table and in the corridors of Westminster palace, fell on deaf ears at the Ministry of Justice.

Employment tribunal fees of up to £1,250 were introduced in July 2013 by the coalition in response to pressure from the private sector, which sought to reduce what were perceived to be “frivolous” claims that cost the taxpayer thousands in legal fees.

Stark figures published by the justice ministry have shown a fall in the number of sex discrimination cases at employment tribunals from 6,310 in the three months before the charges were introduced in July last year to 591 – a 90% fall – during the same period this year. The success rate of the claimants has not significantly changed, suggesting that claims of a high number of frivolous claims under the old system was overstated.

Race discrimination claims are also down 60%, from 1,089 to 422. Overall, 44,334 claims of all types were made to tribunals between April and June 2013. By this spring, the figure was only 8,540 – a fall of 81%.

In recent months the Liberal Democrats have been privately lobbying the Tories to review the potential unwanted repercussions of the fees, which have been described as a “tax on justice” by the TUC.

Grayling’s failure to respond to the Lib Dems’ calls, despite previous assurances that a review would come a year after implementation of the policy, has now so infuriated Cable that he has written to the justice secretary announcing his own inquiry. In a letter to Grayling, Cable wrote: “The fair and effective operation of the employment tribunals system is vital to the successful enforcement of employment rights for which my department is responsible.

“The quid pro quo of my party supporting the Conservative proposal to introduce employment tribunal fees was that we should conduct a rigorous review within a year of their introduction, to determine whether there had been any unwanted consequences and to ensure no one was deterred from legitimate access to justice.

“Despite much prompting from my officials and Jo Swinson [Liberal Democrat minister for employment relations], including at cabinet, 18 months have now passed and nothing has happened. I am concerned that we appear as a government to have reneged on our public commitment to conduct this review.” He wrote: “You gave assurances during parliamentary debates that the Ministry of Justice would monitor the impact of the proposals on women and other vulnerable groups and that the reviews would take place regularly and on an ongoing basis. This is not happening either.

“This review now needs to be progressed as a matter of urgency … I have instructed my officials to initiate a review of employment tribunal fees based on all the publicly available data and research on the impact of fees in employment tribunals.” Cable told Grayling that he would share the findings of his review with the justice department once it was published in a few weeks.

The fees had been welcomed by employers, who face an average cost of £8,500 each time a case is brought.

But they have been branded “a barrier to justice” by Citizens Advice, among others, who found that nearly half of workers with employment issues would have to save for six months in order to afford them.

Citizens Advice has called on the government to align tribunal fees with county court charges in order to widen access. It has also asked for greater promotion of available financial support and more research to assess what measures could be taken to protect employers without deterring legitimate claims.