The civil service faces a "bumpy ride" making Brexit happen while dealing with other priorities, the union leader who represents Whitehall staff has warned.

Dave Penman, from the FDA union, said more resources were needed or ministers would have to rethink other goals.

"Something is going to have to give, and it is not going to be Brexit," he told the Guardian newspaper.

A Cabinet Office spokesman said the civil service was "fully focussed" on getting "the best deal for the UK."

Mr Penman said without more investment in the civil service, some aspects of Prime Minister Theresa May's domestic agenda would suffer.

"It is pure politics that is defining the Brexit debate and forcing May to say this is not a big, difficult job, and it is all in hand," he said.

'Shot down'

"Ministers lack the political courage to admit how complex and time-consuming this will be.

"When anyone pops their head above the parapet - former permanent secretaries, ex-cabinet secretaries, the Institute for Government - and says this is going to take a long time and it's complex, they are immediately shot down and accused of betraying the will of the people."

The Foreign Office and International Trade department were given extra money in last month's Autumn Statement.

The new trade ministry, which is taking over a number of responsibilities from the business department and Foreign Office, is to get an extra £79.4m over the next four years, while the Foreign Office was given extra money to recruit trade policy experts within its diplomatic network.

'Unique complexity'

However, other departments face budget cuts at a time when implementing the decision to leave the EU is likely to increase their workload.

In the Guardian interview, Mr Penman - general secretary of the First Division Association (FDA) which has 19,000 members - said the civil service was used to coping in a challenging financial environment but suggested the "unique complexity" of Brexit was likely to put a strain on Whitehall.

"The civil service is either going to have to be given more resources to deal with Brexit and its usual work or it will have to change its priorities," he said.

"And government doesn't want to admit to either."

'Formula One car'

He highlighted the Department for Environment, Food and Rural Affairs as one department to face difficult choices because of the amount of EU legislation affecting the farming and fishing industries and the level of financial support they received from Europe.

"The civil service will have to effectively run a Formula One car whilst building next year's car at the same time. It can be done, but it's going to be a bumpy ride."

Under plans set out in last year's spending review, departments whose budgets are not protected are expected to save almost a third of their day-to-day spending by 2020

Downing Street has said the full "machinery of government" is being put behind the Brexit process, with two new departments created and leading civil servants being redeployed across Whitehall.

A Cabinet Office spokesman said "The civil service is fully focused on delivering the government's commitment to leave the EU and get the very best deal for the UK.

"We are equipping ourselves with the right people and the right skills across government to make this happen."