The UK receives tens of thousands of asylum applications ever year. Usually less than half are found to be valid, even at the end of lengthy appeal processes, and yet only a minority of those subsequently leave the country. As a result there is a mounting backlog of illegal immigrants waiting to be removed. Most never will be. David Wood, Theresa May's former Director General of Immigration Enforcement at the Home Office, here sets out the challenge of maintaining Britain's border controls and shows how the system largely fails to deal with those who are here illegally. One of the central difficulties revolves around the asylum system, and the scope for its abuse by those who are not refugees but submit applications as a last-ditch bid to avoid deportation. This risks overwhelming resources and lengthening the time it takes to process the claims of genuine asylum seekers who are fleeing persecution and war. It also helps undermine voters' trust in the system and fuels anger that the rules are not enforced properly. `It is essential that the UK's asylum system is nothing but supportive of those who are genuinely fleeing persecution,' Wood writes. `But where asylum processes are being used as a way of facilitating economic migration it is essential to be able to quickly and efficiently distinguish between the two, in order to ensure those entitled to help receive it quickly, and to ensure that UK citizens do not lose faith and support for a system that is rife with abuse. `It ought to be possible to do better in enforcing immigration rules than we have been doing, and that must start with a better understanding of the challenges we face.'