The system is simple.

Etihad will fly passengers out of Kuala Lumpur direct to its home base in Abu Dhabi. As a transit passenger, no UAE stamp is put into their passport. As a UAE airport, flights from Abu Dhabi are banned from the UK.

Etihad flies its passengers to Dublin where flights from the UAE are permitted.

But it is not a through ticket. It is known as a "self-transfer" which means that passengers must clear immigration in Dublin, retrieve their baggage - and then check in again for the second sector i.e. the flight from Dublin to London. And it's on a different airline: Aer Lingus.

This means that passengers for London can use the much less restrictive immigration controls if they are UK or Irish citizens. And it means that they are now flying from Dublin not from the UAE and therefore not subject to the ban which, it must be noted, is on flights not on people. This means that those who choose to lie about where they have been in the past ten days or so can do so.

Those travelling from Malaysia are coming from an amber list country via a red list country into a green list country via a transit hub.

If they arrive in the UK without declaring that Abu Dhabi transit, they can enter self-isolation for ten days. They still need to take two PCR tests within that period. If they were travelling from, say, Singapore using the same route and also lie, they would be travelling from a country on the green list and that means no isolation and one PCR test.

If they do declare their Abu Dhabi touch point, they must spend at least ten days in designated quarantine hotel and take two PCR tests, the total cost for which is about GBP2,000. The incentive to cheat is therefore significant.

Of course, the obvious solution is for transit passengers to have their passports stamped but that's expensive, complicated and time consuming and short transits would become impossible.

The airline is also laundering passengers through Munich where, ironically, passengers change to British Airways.

Emirates is flying passengers via Dubai then Zurich where they change to Swiss en route for Heathrow. Routes via Zurich are also popular for flights from the Far East which do not hub in the UAE.

However, passengers arriving in the UK from the EU or Switzerland do go through more formal immigration than those arriving from Ireland.

As Singapore's airport, Changi, is demonstrating, airport hubs are a potential petri dish for infection. The question here is how high risk that is and whether passenger laundering will present a serious risk to the UK.