A leaked memo suggesting that Theresa May's administration has no plan for Brexit has been disowned by ministers, with a spokesman saying it has "nothing to do with the Government at all".

The document, 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.

According to the newspaper, the memo was written by a consultant working for the Cabinet Office, who had identified "well over 500 projects" being undertaken by Whitehall departments to implement Brexit, creating the need for up to 30,000 extra civil servants.

Read more: Leaked Cabinet Office memo - Theresa May's Government has no Brexit plan

But a Government spokesman said the document was "unsolicited" and had no authority.

"This unsolicited document has nothing to do with the Government at all, said the spokesman. "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."

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.

Read more: Leaked Cabinet Office memo - Theresa May's Government has no Brexit plan

The Times memo, dated November 7 and entitled "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.

Despite extended debate among senior civil servants, "no common strategy has emerged" and "departments are struggling to come up to speed on the potential Brexit effects on industry", it said.

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.

And Labour MP Pat McFadden, of the Open Britain campaign, said: "This report shows how totally unprepared the leaders of the Leave campaign were for winning the referendum.They urged the UK to leave without having any real idea of what it would involve."

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.

Read more: Leaked Cabinet Office memo - Theresa May's Government has no Brexit plan

"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, we will not seek to block or delay it.

"To do so would put Labour 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."

Mr McDonnell said it is a "dereliction of duty" that the Government has not hired enough Whitehall staff to deal with Brexit just months away from triggering Article 50.

Answering questions after a central London speech, the shadow chancellor said: "If we need the staff we need to employ them, whether it's 30,000 or below.

"I just find it staggering that we have a Government facing a momentous decision on the implementation of negotiations and they haven't even staffed up.

"It's just extraordinary, it's a dereliction of government duty."

He also criticised ministers for putting their careers ahead of the interests of the country.

"It isn't just about staffing, the memo is confirming what we knew, which is ministers arguing amongst themselves all the time, no common view of the way forward and no shared vision of the future for our country," he said.

"It's just dismal what's happening at the moment, to actually start putting their individual careers or their individual ideologies before the interests of the country is unacceptable."