Britain's top civil servant has warned that Brexit has added to Whitehall's already heavy workload and that there was a need to be realistic over its timetable as it supports the ideas and agenda of the new government under Prime Minister Theresa May.

Chief executive of the civil service, John Manzoni said that Brexit is probably "the hardest thing any of us have had to deal with."

Addressing a seminar on pay, reward and performance management last week, Manzoni said the government already had too many objectives before the UK voted to leave the EU and that it now faced the prospect of having to re-plan and re-prioritise, the Civil Service World reports.

"When I look across from outside, I say we're doing 30% too much to do it all well — that is the nature of government," he said.

Adding to speculation that the government still has no Brexit strategy in place, Manzoni said that the civil service is "still in thinking mode" over Brexit. He noted that under May's stewardship, two new departments have been created and two others have been merged.

The UK and EU flag Adam Berry/ Getty Images

There is now "an enormous amount of work going on across the civil service redefining what the future of the various policies might be, or what we might like them to be."

He highlighted the fact that the reorganisation of the Department for the Environment, Food and Rural Affairs, whose policy agenda will be heavily affected by the UK's withdrawal from the EU's Common Agricultural Policy, was already taking up a great deal of senior leaders' time.

"Along comes Brexit — 90% of your business has changed. How do we absorb that? The fact is we need to go back, we need to re-plan, we need to be realistic, we can't do it all — it won't all happen within the existing envelope."

Meanwhile The Times reports that the latest figures from the government show that two-thirds of civil servants at the Brexit department do not think that they have "clear work objectives."

Only 38% of civil servants at the Department of Exiting the European Union agree that they have clear objectives — the lowest score of any department. It said the Department for International Trade also recorded poor scores in the annual civil service people survey.

In fact, 71% of the civil servants at the trade department do not agree that Brexit "as a whole is managed well."

The Times quoted a spokesman for the Brexit department as saying: "The last six months have seen a transition from one government to the next, and new departments are being created to reflect the priorities and agenda of the new prime minister."

Government still has no strategic plan on Brexit talks

Whitehall is overstretched by Brexit preparations. Getty

There are now concerns over whether May's target of triggering Article 50 in the first quarter 2017 can be met. A leaked memo from a consultant claimed that the government had no clear Brexit strategy and that the Cabinet was split on key issues.

The UK's former permanent under-secretary at the Foreign and Commonwealth Office, Sir Simon Fraser cautioned that the government did not have a central plan for Brexit four months before its trigger deadline.

"It is indeed proving to be a very considerable challenge in Whitehall to do this [pull together a plan for the negotiations]. The government is still in information-gathering mode and is not yet at the point of integrating that into a central plan. And that I assume will have to happen before the triggering of Article 50 next year," he told the Brexit select commitment.

He warned: "Not enough progress has been made - more needs to happen and time is relatively short."