Downing Street appears to have a credible Brexit proposal after a week which began with a Number 10 aide calling a deal "essentially impossible".

The proposal thrashed out between Boris Johnson and Leo Varadkar, in theory, deals with the main of the outstanding intractable issues.

The nascent plan, revealed by leaks to Sky News and others, appears clever because it creates the illusion of victory for both sides on the most difficult issue of all – customs.

Under the plan, the whole of the United Kingdom leaves the EU customs union, in a big and important win for Boris Johnson.

However, the EU tariff regime will continue to be applied on the whole of the island of Ireland.

This means that the tariffs charged in Great Britain could be different to those in Northern Ireland.

But under the compromise agreement, all businesses in Northern Ireland will be able to benefit from UK tariffs by offering a rebate on goods sold, if the UK tariff is lower than the EU one.

This plan allows Europe to say the island of Ireland is in one customs zone.

Which is a win for them.

This plan also allows Mr Johnson to say the whole of the UK has left the customs union and Northern Ireland can – like the rest of Great Britain benefit from trade deals, which is a win for him.

This compromise comes in the spirit of the Good Friday Agreement, which allowed for competing interpretations of one set of rules to allow peace in Northern Ireland.

Though, in truth, Britain has probably compromised more than the EU in this process.

Putting together the various compromises offered by Mr Johnson, Northern Ireland will, under Mr Johnson’s plan, now be in the same regulatory and agricultural zone and subject to the same tariffs as the rest of the EU.

There will be a regulatory and agrifoods border in the Irish Sea and potentially some customs checks too, albeit while ensuring that Northern Ireland businesses do not lose out financially from the arrangement.

Some have compared the plan to a version of Theresa May’s Chequers deal, but for Northern Ireland.

Others call it a Northern Ireland backstop – rejected decisively by MPs time and again – in all but name.

Number 10 will reject both labels and probably call it a free trade zone.

There will also be the option for Northern Ireland – either via Stormont or another mechanism – to vote on the plan in a few years, details of which are yet to be spelt out.

Only Mr Johnson could have compromised like this, and – as of now – still appear to keep both the DUP and Brexiteers on side.

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However, the government is not home and dry.

They need to convince the rest of the EU, and there may be reservations about whether the UK post-Brexit can be trusted to administer the EU external border, and whether they are prepared to tolerate the risk of smuggling.

Then they also need to convince parliament.

Mr Johnson does not have a majority and, since he expelled a number of Tories who were voting to block no-deal, his numbers have been going backwards.

Several have joined the Lib Dems, while others who support a second referendum are not minded to support Mr Johnson either.

Meanwhile, Mr Johnson’s Brexit plan paves the way for Great Britain to be subject to the hardest possible Brexit, potentially allowing for a reworking of the entire economic and social model towards low-tax low-regulation states with small safety nets, such as Singapore.

This makes it harder for Labour MPs to support.

This battle is not over yet.

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(c) Sky News 2019: Johnson’s Brexit proposal creates illusion of victory for both sides