What is Boris Johnson’s offer?

The proposal is similar to Theresa May’s deal but with the Irish border backstop ripped up and replaced by his own plan, labelled “two borders for four years”.

Under the plan the entire UK would leave the EU on 31 October with the original transition period staying in place until 31 December 2020.

On the new Brexit day of 1 January 2021, Great Britain would leave all the institutions and structures of the EU including the customs union, which allows frictionless trade with all member states and all regulations on goods and food. It will also mean the UK exiting judicial structures including the European court of justice, security and defence arrangements and agreements such as data sharing.

What about Northern Ireland?

Northern Ireland would also leave the customs union on 20 December 2020. But it would remain aligned on regulations and, it is assumed, freedom of movement of people within the island of Ireland for another four years, until 2025.

But would that not mean a border down the Irish Sea?

Yes. Goods coming into Northern Ireland would have to be checked to see if they complied with EU standards, which would apply across the island.

Quick Guide Why is the Irish border a stumbling block for Brexit? Show 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

Was this not a red line for the DUP?

Yes. But the Democratic Unionist party has said this is a pragmatic compromise to help get a deal over the line.

What about the Irish border?

If Northern Ireland maintains EU regulations on food and manufactured goods and freedom of movement, there would be no need for any checks on the Irish border. This is why Johnson thinks it is a winner.

Is this not a time-limited backstop?

Yes. But the DUP believes it will work if after 2025 new structures are put in place to give Belfast and Dublin parliamentarians oversight of how Northern Ireland moves from there, with the option of remaining aligned to EU regulations a possibility. They see the restoration of the British-Irish ministerial council as vital to this, giving representatives from the Stormont assembly (assuming it is up and running again) and the Irish parliament a say on any changes of regulations on food and manufactured goods post-2025.

And would that be acceptable to the Irish government and the EU?

At first sight, no. Infrastructure on the Irish border even after an effective transition period of four years is a red line for Ireland, which is concerned it would destabilise border communities that have flourished economically and socially since peace 21 years ago.

When will we know if the deal is acceptable?

The deadline for a deal is in 10 days’ time. This will give the European commission enough time to circulate the deal among EU member states for their approval or rejection before the next EU summit on 17 October. Normally, papers that are formally discussed are circulated one week before.

What do those behind the deal say?

The DUP says it has compromised, now Dublin and Brussels should budge too. Johnson has said the EU needs to show more flexibility.

What if the EU rejects the offer?

Under the Benn Act passed by MPs just before Johnson prorogued parliament, the prime minister must send a letter seeking an extension to article 50 until the end of January. This letter must be sent by 19 October.