Brexit planners say they have been left frustrated and despairing after receiving a confidential eight-page memo from Whitehall that gives no new details or specific guidance about how the UK is preparing for its departure from the EU.

The document, leaked to the Guardian, states the “UK will be leaving the EU on 31 October whatever the circumstances”, but acknowledges there is still a mountain to climb to prepare people and businesses for a potential no-deal Brexit.

Though time is running short, the memo makes clear that the government is not yet ready to share its latest “reasonable worst case scenarios” – which help local emergency planners to focus their resources.

However, it admits only half the population think the UK will actually leave the EU at the end of this month, and 42% of small businesses “are unsure of how they can get ready”. Less than a third of the British public have “looked for information on how to prepare for Brexit”, it adds.

The document says the government “will ramp up activity for key target audiences”, but does not specify when.

Quick guide Why is the Irish border a stumbling block for Brexit? Show Hide Counties and customs Inside the EU, both Ireland and Northern Ireland are part of the single market and customs union so share the same regulations and standards, allowing a soft or invisible border between the two.

Britain’s exit from the EU – taking Northern Ireland with it – risks a return to a hard or policed border. The only way to avoid this post-Brexit is for regulations on both sides to remain more or less the same in key areas including food, animal welfare, medicines and product safety. The 'backstop' in Theresa May's Withdrawal Agreement was intended to address this - stating that if no future trade agreement could be reached between the EU and the UK, then rules and regulations would stay as they are. This has been rejected by Brexit supporters as a 'trap' to keep the UK in the EU's customs union, which would prevent the UK striking its own independent trade deals. There are an estimated 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, and about 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. Brexit supporters say this can be managed by doing checks on goods away from the border, but critics say it will be difficult to police this without any physical infrastructure like border posts or cameras, which could raise tensions in the divided communities of Ireland.

Interactive: A typical hour in the life of the Irish border Photograph: Design Pics Inc/Design Pics RF

The “not for publication” document was sent on 26 September to council leaders and the heads of 38 local resilience forums, which are supposed to help communities prepare for the fallout from a no-deal Brexit.

But the lack of detail has left many planners exasperated and fearing the government does not want to tell LRFs its main concerns, for fear they may stoke public concern about the effects of leaving the EU without a deal.

At a point when the government needed to be straight about the potential problems, it was hunkering down and keeping secrets, said a senior source.

“People need information now, not later. Time is desperately short and we are still being kept in the dark.

“What are we supposed to be planning for? We need to know the potential impacts, but they haven’t told us what to look out for. We cannot make plans if they don’t tell us. It’s all political smoke and mirrors. This document indicates the government has no grasp on the details of the response.”

Another source said mixed messages from the government, and the lack of clarity from ministers, was making emergency planning on the ground incredibly difficult.

“There is a sense of frustration. Everything is getting more and more complicated. We still don’t know what the hell is happening. Nobody does. Nothing has been ironed out.”

The Local Government Association said its members were working “tirelessly” to prepare for the end of October, but that they needed more guidance from the centre.

“There remains information and advice gaps that councils are facing while helping their communities prepare, which need to be met by the government,” said Kevin Bentley, chairman of the LGA’s Brexit taskforce.

“Councils also need certainty to plan for their communities over the longer term, such as on the domestic replacement for EU funding.”

The document was prepared by the Ministry of Housing, Communities and Local Government (MCHLG) and is titled “HMG Top Lines”.

Though it was prepared by civil servants, it begins by stating categorically: “The UK will be leaving the EU on 31 October whatever the circumstances. If it is not possible to reach a deal we will have to leave with no deal.

Q&A What is a customs union? Show Hide A customs union means that countries agree to apply no or very low tariffs to goods sold between them, and to collectively apply the same tariffs to imported goods from the rest of the world. International trade deals are then negotiated by the bloc as a whole. For the EU, this means deals are negotiated by by Brussels, although individual member state governments agree the mandate and approve the final deal. The EU has trade deals covering 69 countries, including Canada and South Korea, which the UK has been attempting to roll over into post-Brexit bilateral agreements. Proponents of an independent UK trade policy outside the EU customs union say Britain must forge its own deals if it is to take advantage of the world’s fastest-growing economies. However they have never explained why Germany manages to export more than three times the value in goods to China than Britain does, while also being in the EU customs union. Jennifer Rankin

“The government is committed to preparing for this outcome. We are better prepared than many think but not as well prepared as we can be.”

The document states: “We will ensure that there is as little disruption to national life as possible, whatever the circumstances after Brexit. With the kind of national effort our people have made before – and will make again – we believe that is possible.”

It goes on to say that the government is “undertaking a review of the reasonable worst-case scenario planning assumptions … and will publish the revised version in due course”.

Aside from the lack of detail or explanation about how this would be achieved, some planners are said to have been riled by the language being used, saying it seemed overtly and unnecessarily political given it was prepared by civil servants rather than ministers.

Whitehall sources said it was drafted to give emergency planners an insight into government thinking.

A spokesperson for MCHLG said: “We don’t comment on leaks. We’re working very closely with local resilience forums to support them in their preparations to ensure we will be fully ready for Brexit on 31 October, whether we leave with or without a deal.”