Here’s a chastening little exercise for supporters of Boris Johnson: go to the YouGov weekly tracker of the most important issues facing the country, click on the sub-sample of Leave voters and then press the “hide trend line” button that takes away statistical smoothing so you can see a bang up-to-date snapshot.

You will see that as of 22 November, Immigration & Asylum is far and away the leading concern of this half of the electorate – basically the half that delivered the Tories their landslide election win in 2019.

The voters know that waiting for France to fully cooperate is like waiting for Hell to freeze over

Some 60 per cent rate it as one of the top three most important issues facing the country, far ahead of the 45 per cent each that Health and The Economy rack up. And it is on an accelerating upward path. Given the Channel drownings on November 24, it would be no surprise to see it hit 70 per cent next week.

As Tory MPs have been corroborating direct from their postbags, this is clearly the issue that is most responsible for the ebbing popularity of the Government — far more so than the endless sleaze rows that have dominated the broadcast media’s coverage of politics in recent weeks.

The “dog ate my homework” excuses of Johnson and Home Secretary Priti Patel for their failure to get a grip on the cross-Channel chaos have stopped working with their own bedrock support. These voters have clocked that waiting for France to fully cooperate in stopping the traffic is like waiting for Hell to freeze over.

So they are propelling Johnson’s regime into a new political phase, one in which it will either embrace radical and controversial measures to tackle the issue or see the electoral coalition which has sustained it collapse.

In a House of Lords debate on the issue on Thursday, Lord Green of Deddington, who as Sir Andrew Green founded the MigrationWatch pressure group, had a sobering assessment for ministers: “The numbers arriving in small boats have trebled this year, compared with last year, and are now 25,000 with five weeks to go until the end of the year. On present form, the numbers could treble again next year…The contrast between the slogan ‘Take Back Control’ and the present situation could hardly be starker.”

Over the past 15 years or so there has been no more accurate forecaster of migration trends than Lord Green and his think tank. It was, for example, they who rubbished the Home Office’s ultra-low predictions of the numbers who would come from Eastern Europe after EU enlargement.

Ministers would therefore be wise to have as their working assumption that in the absence of decisive change 80,000 will arrive illegally across the Channel next year. With the Leave-voting tribe already aflame with indignation, such an outcome does not bear thinking about for long — certainly not for any Tory MP with a majority below 10,000.

So what measures are open to ministers that do not depend on the active cooperation of France and other EU nations, an unreliable commodity that can be withdrawn as quickly as it is bestowed?

Withdrawing from the Geneva Convention would cause uproar: hundreds of doses of smelling salts would be required for all the peers who would pass out with apoplexy and indignation

A clue was provided in this same Lords debate by Tory former Cabinet minister Lord Lilley. Predicting the likely failure of Patel’s much-trumpeted Nationality & Borders Bill to sort the issue out, he declared that decisive progress would “be possible only if we, together with other like-minded countries, insist on renegotiating and, if need be, resiling from the Geneva Convention and replacing it with something relevant to an age of mass movement”.

This idea was contained in the 2005 Tory election manifesto, when Michael Howard was party leader and Lilley a key shadow cabinet member. That document had the following to say: “On asylum, a Conservative Government will not allow outdated and inflexible rules to prevent us shaping a system which is more humane, more likely to improve community relations and better managed. So we will take back powers from Brussels to ensure national control of asylum policy, withdraw from the Geneva Convention and work for modernised international agreements.”

Yet restoring such a policy now that the Tories are in power would undoubtedly cause uproar among the liberal left establishment that has such a grip over UK public life and much political media. Hundreds of doses of smelling salts would probably be required just for all the peers who would pass out with apoplexy and indignation at hearing the words Geneva, convention and withdraw being uttered in the same sentence again.

Having been scarred by all the establishment flak he had to fly through to win the battle for Brexit in 2019, does Boris Johnson have the appetite for another epic struggle to restore national democratic sovereignty over territory farmed out to international bodies long ago? Certainly he can wave goodbye to restoring his reputation among his “liberal Tory” social circle if he takes on this mission – net zero or no net zero.

Yet withdrawing from a thicket of international agreements — not just the Geneva Convention on Refugees, but probably the European Convention of Human Rights and the jurisdiction of its Strasbourg-based court and possibly the much more recent UN Global Compact for Migration too — may well be necessary in order to implement a tough new asylum system which embraces standard offshore processing and moves long-term UK residency beyond the reach of irregular migrants.

Should Johnson take on this challenge, it will turn into the dominant political battle of the second half of this parliament and loom very large over the next general election too. Should he duck it,  it is hard to see him making it that far.