A memo suggesting Theresa May's administrat ion has no plan for Brexit was not commissioned by the Government and has "no credence", the Prime Minister's official spokeswoman has said.

The document, prepared by accountancy firm Deloitte and obtained by The Times, warned that Cabinet splits were delaying the Government's ability to agree a negotiating strategy, which may not be ready for six months - after Mrs May's deadline of March 2017 for the launch of withdrawal talks with the EU.

It identified "well over 500 projects" being undertaken by Whitehall departments to implement Brexit, creating the need for up to 30,000 extra civil servants.

But a Government spokesman said: "This unsolicited document has nothing to do with the Government at all. It was produced by an individual from an external accountancy firm. It has no authority and we don't recognise any of the claims it makes. We are getting on with the job of delivering Brexit and making a success of it."

Mrs May's spokeswoman said the Prime Minister had not been aware of the existence of the document, which appeared to be an attempt by Deloitte to pitch for Government work.

"This individual has never been in Number 10 or engaged with officials in Number 10 since the Prime Minister took office," said the spokeswoman.

"It is really for Deloitte to answer what it is about. Certainly it has not been commissioned by Government, it's nothing to do with us. It hasn't been distributed widely across Government. It does seem as though this is a firm touting for business.

"The Prime Minister is very clear on the timetable and I struggle to understand how one individual who has never met the Prime Minister or any members of her team can then decide that the timetable is somehow off course or different."

There was no immediate response from Deloitte to reports of its involvement.

The row came as reports emerged that Brussels may demand up to 40-60 billion euros (£34-52 billion) as an "exit bill" from the UK, to cover unpaid budget commitments, pension liabilities, loan guarantees and spending on UK-based projects.

The Financial Times said the sum - equivalent to around three to five years' worth of UK contributions to EU budgets - was part of a negotiating position being drawn up by the European Commission ahead of formal talks under Article 50 of the EU treaties, expected to lead to Brexit in 2019.

Commission chief Brexit negotiator Michel Barnier was reported to be pushing for a draft exit deal by mid-2018 under a narrow "divorce-first" negotiating stance, which would defer detailed talks on future trading relations until after the terms of departure are agreed.

Under this plan, transitional arrangements could be put in place to cover the period of five or more years until a trade deal is sealed, said the FT.

The Deloitte memo, dated November 7 and ent itled Brexit Update, warns that while each Whitehall department has developed Brexit plans, these fall "considerably short" of a "Government plan for Brexit" because of the lack of prioritisation and an overall negotiation strategy.

It criticises Mrs May for "drawing in decisions and details to settle matters herself", and warns that big companies will "point a gun at the Government's head" after Nissan was given assurances about trading conditions once Britain leaves the EU.

And it identifies a split between the "three Brexiteers" - Boris Johnson, David Davis and Liam Fox - on one side and Chancellor Philip Hammond and Business Secretary Greg Clark on the other, which it claims is holding back agreement on a plan.

A senior civil servants' union warned that the memo appeared to confirm ministers were hoping to implement Brexit "on the cheap".

"Whilst politicians squabble about hard and soft Brexit, there is a deafening silence from ministers over whether any additional resources will be provided to deliver this momentous task," said FDA chairman Dave Penman.

"Brexit on the cheap appears to be the Government's preferred approach, but this will satisfy no-one. Next week's Autumn Statement is the Government's opportunity to outline how it will provide the resources the civil service needs to ensure a successful Brexit."

Liberal Democrat leader Tim Farron said the memo had exposed a "shambles" at the heart of government, while SNP Europe spokesman Stephen Gethins said it offered a "staggering insight into the utter confusion and chaos" within the upper ranks of the Conservative Party.

Transport Secretary Chris Grayling, who sits on the Cabinet's Brexit committee, dismissed suggestions that the civil service would have to recruit tens of thousands to deal with the workload associated with unravelling the UK's EU membership.

"I don't know what 30,000 people would do in this process," Mr Grayling told BBC Radio 4's Today.

Meanwhile, Mrs May received a boost, as Labour pledged not to block or delay the invocation of Article 50 and called for a "more positive" view of Brexit.

In a speech in central London, shadow chancellor John McDonnell said: "We must not try to re-fight the referendum or push for a second vote. And if Article 50 needs to be triggered in Parliament, Labour will not seek to block or delay it.

"To do so would put us against the majority will of the British people and on the side of certain corporate elites, who have always had the British people at the back of the queue."

Labour should "embrace the enormous opportunities to reshape our country that Brexit has opened for us", said the shadow chancellor, adding: "In that way we can speak again to those who were left behind and offer a positive, ambitious vision instead of leaving the field open to divisive Trump-style politics."

Shadow Brexit secretary Sir Keir Starmer said: "Whatever the purpose of this memo, it is absolutely right about two things. First, the Government are in a mess over Brexit and do not have a plan.

"Second, the way Theresa May is handling this process shows that, just like her predecessor, she is putting the interests of her party before those of her country."

Europhile Tory former Cabinet minister Ken Clarke said "the Government at the moment does not have an agreed strategy" on Brexit.

"That's not surprising, that's not a criticism. It wasn't considered by people on either side of the argument in the run-up to the referendum, nor was it debated in the run-up to the referendum," he told BBC Radio 4's World at One.

He said it would take "a good six months to get the right policy which minimises the damage and actually deals with the real world of how we can best manage our future relationships" with the EU and the rest of the world.

Former Cabinet minister Sir Oliver Letwin said civil servants would see Brexit as a "huge career opportunity" but questioned whether more officials were needed.

"My guess is some of the brightest and best civil servants, many of them, who come in with an interest in the most important things this country is doing will see this as a huge career opportunity," he told the World at One.