Shuttle diplomacy: exit checks will cause extra headaches at Eurotunnel's terminal / AFP/Getty

The man who pays his way

From Wednesday, the Government will know when you are on holiday. Possibly. Exit checks on departing travellers were dropped 17 years ago as an inefficient use of resources of little use to the "integrity of the immigration control". But they make a comeback from 8 April. Apparently everyone is now in favour of keeping tabs on who's departing the UK, just as the old East Germany used to be frightfully keen on knowing who was going west through Checkpoint Charlie.

You will have noticed that an election campaign is under way. You will also have noticed that immigration is regarded as a key issue by many politicians and some voters. If you were paying particular attention in 2010, you will also recall the Coalition promised that checks on who is leaving the UK would be instituted within the lifetime of this Parliament.

A pedant might observe that the promise has technically been breached, because the Queen dissolved Parliament on Tuesday. But four weeks before the eve of the next election, exit checks are finally taking effect.

So what will you notice? When travelling by air, nothing. For several years airlines have been obliged by the Government to collect advance passenger information: name, place of birth, passport number, childhood nickname, breed of first pet, favourite member of One Direction, that sort of thing. Even though some airlines warn that you will not be allowed on board unless you have diligently filled in the right online form before you arrive at the airport, I have seen no evidence that this threat has ever been applied, nor the information thus gathered has been used by the Government. But the fact that it's been religiously collected means it is a piece of cake for airlines to pass on the details to the Home Office.

Eurostar passengers will be scratching their travel-weary heads when they board a train to Paris or Brussels from London St Pancras. Your papers will be checked by the same security team who scan your belongings for dangerous items. Five seconds after they hand back your passport, you hand it to a French immigration officer so they can scrutinise the same information. It appears to be too politically difficult for the UK Government to ask the French to keep tabs on who's leaving.

The ferry firms know who's on board, because of long-standing European rules demanding a passenger manifest.

Exit checks create the biggest headache for Eurotunnel, traditionally a manager of traffic rather than of passengers. The firm is in the business of maintaining smooth-flowing links between Folkestone and Calais for cars, trucks and coaches. The currency is vehicles, not people. Eurotunnel is adroit at loading shuttles and using number-plate recognition to check vehicles travelling through. But until now the firm has had no interest in knowing who is actually inside. Suddenly, Eurotunnel is obliged to collect the identities and nationalities of up to nine people in a car and 60 in a coach. Sure, the French authorities have a presence at Folkestone, just as they do at St Pancras in London. But as anyone who has used these links will know, checks for entering France vary from cursory to non-existent.

The imitation game

At this point I confess I lent a colleague a passport when he could not find his just before he was due to go on a motoring holiday to France, on Eurotunnel from Folkestone to Calais. "If you're asked, just wave it at the French," I advised. Simon (as his name happens to be) was not checked at all on the journey to France, and on the way back showed his driving licence to the British passport official and was allowed in after a mild ticking-off.

Couldn't happen with exit checks, could it? Well, yes, it could.

"Data will be gathered on all passengers," says the Home Office. But when you look more closely at how it is to be done, you realise that the system is full of holes.

The people of particular interest are non-EU citizens who have overstayed, who will want to conceal this fact; and those who are legally on holiday or studying in Britain, but who hope to overstay. The first group want to leave the country undetected; for the second it is convenient to appear to have departed. Loopholes exist for both these constituencies.

Buses are treated differently at Eurotunnel's Folkestone terminal because of the sheer volume of passengers. School coach parties of under 16s will be waved through unchecked. Everyone else has to troop off the bus and pass through a hut where their passport details are scrutinised. Then they can go into the passenger terminal and enjoy a coffee or a snack at the upmarket fast-food provider, Leon. Should the bus passengers fall into conversation with car drivers, they will discover that their new pals have not been checked. Drivers provide identity details online or by phone, but have not yet passed through the checkpoint at which one in five EU cars is to be examined.

As some of the more chaotic frontiers of Latin America so entertainingly demonstrate, having checked and unchecked people mingling at borders makes it impossible to know who's coming or going.

Checkpoint Charlies

Because of the haphazard way checks are being implemented, you need not be a people-smuggling criminal mastermind to identify a way to be counted out of Britain without actually leaving these shores. Equally, overstayers who want to try to leave the country without being noticed have an 80 per cent chance of avoiding detection. So when the Home Office says that "exit checks data will help us to target people who have overstayed their visas and are in the UK illegally," the politicians may be fooling themselves, as well as us.