Lord Kerslake, the UK's former chief civil servant, has raised concerns that Britain's civil service is not in a "fit state" to negotiate Brexit.

He said he was "seriously concerned" about the capacity of UK's civil service to negotiate Britain's exit from the European Union. He added that leaving the economic single market would only further exacerbate the issue.

Speaking to Sky News at the launch of his report into the Treasury, Lord Kerslake, who led the civil service from 2012 to 2014, slammed former chancellor George Osborne and the Treasury for their extreme economic warnings ahead of the referendum.

Yet he reserved his most dire prognostications for Whitehall's possible inability to handle the Brexit negotiations amidst continuing civil service job cuts.

"I am seriously concerned a proper assessment of the skills and resources needed hasn't been done. In fact, Whitehall is still reducing staff quite dramatically" he noted.

Lord Kerslake continued: "And you do have to ask the question how do you both deliver Brexit and have a significant domestic policy agenda as well?

The UK and EU flag Adam Berry/ Getty Images

"If the government wants to have both a strong economic mandate — domestic policy being developed afresh — and deliver Brexit, you're going to have to resource that and think very carefully about reductions you're making."

Oliver Ilott, a senior researcher at the Institute for Government, agreed with Lord Kerslake.

"Whitehall is smaller than it's been for many decades and across Whitehall, about 20% of people have gone since 2010. But that hasn't been even — in some departments it's been much heavier than that," he said.

Ilott said that in the Department for Environment, Food and Rural Affairs (Defra), about 35% of its staff were laid off, despite the fact that 80% of its work relates to Brussels.

"So it's got a huge amount of Brexit work to do and at the same time it's got the same list of domestic policies that it needs to deliver that it was given back in 2015, and so for departments like Defra, there is pressure beginning to build," he said.

Dave Penman, general secretary of the First Division Association, which represents public servants, said that that the civil service had risen to the challenge but warned that the government "has to commit to the civil service as well."

He said the government has to give the civil service the necessary resources and the capability that's required. "And that may mean bringing skills from outside and it also means supporting the civil service at a time when it is asking it to cut its resources, as well as dealing with the additional demands of Brexit."