Senior civil servants have called for a pay rise to deal with “unsustainable” Brexit workloads. The First Division Association (FDA), which represents senior civil servants and diplomats, said their pay does not “remotely compare with the wider marketplace”.

The union said officials could not do their work to the standard they would like due to an untenable workload and “inadequate” government resources. The challenge of working on leaving the European Union while delivering existing government priorities has pushed the civil service “to its limits”, with officials working more hours for falling real-terms pay.

In a survey submitted as evidence to the senior salaries review body (SSRB) by the FDA, 94% of respondents said their pay framework was “not fit for purpose”.

It comes as the row over Whitehall’s readiness for Brexit rumbles on after the resignation of Sir Ivan Rogers as the UK’s ambassador to the EU. FDA general secretary Dave Penman said: “Fresh from reducing the size of their departments, the SCS [senior civil service] is now tasked with implementing Brexit on top of all of the government’s existing priorities.

“Some haven’t seen a pay rise in a decade; all have seen their pay cut in real terms by around a quarter. The strain of the pay freeze and staff reductions is taking its toll. The previous chancellor’s policy of public sector pay restraint has led to a demoralised workforce and a civil service now reliant on expensive contractors and salary premiums for new hires.

“This chancellor needs to take a more realistic position and heed the FDA’s call for real investment in the SCS, not a never-ending series of temporary fixes dreamed up on the hoof that end up costing the public more than before the pay restraint began.”

But former Tory leader Iain Duncan Smith told the Daily Mail: “This is total nonsense.

“Senior civil servants should be embarrassed that their union is asking for a pay rise at a time when the entire public sector is subject to necessary pay restraint. These officials are simply being asked to do their job. They should be relishing the challenge – and the best ones will be.”

Steve Baker, chairman of the influential European research group of Tory MPs, said: “For normal people earning normal wages these civil servants already look very well rewarded.”