As part of David Cameron's "Save Nick" operation, the coalition has brought forward a series of policies designed to stamp the Lib Dems' identity on the government.

We've already been promised a new "crackdown" on tax avoidance and a scholarship fund for poorer students. Today, Clegg will confirm details of the government's plan to end the moral outrage of child detention – one of the Lib Dems' key manifesto promises and a policy we've long argued for through our No Place for Children campaign.

Clegg is due to announce that no children will be detained in asylum centres this Christmas and that the practice, which he described as "state-sponsored cruelty", will end altogether by 12 May – the first anniversary of the coalition agreement.

The accompanying statistics are a grim reminder of Labour's shameful treatment of asylum-seekers. In the party's last term in office, on average, almost seven children a day were locked up; 173 children were detained for longer than a month in the last year alone. In total, 7,075 children were locked up for an average of 13 days.

One study revealed that 65 per cent of children had suffered physically due to their detainment and that more than half had been damaged psychologically, the symptoms including heightened anxiety, loss of bowel control, refusing food and bedwetting.

Clegg will say:

Because our starting point is this: there is no greater test of civilised society than how it treats its children. Today's announcement marks a big culture shift within our immigration system. One that puts our values – the protection of children – above paranoia over our borders. One that prioritises doing the right thing [rather] than looking and sounding tough.

With this in mind, there seems little reason why the coalition should not end child detention immediately. There is every risk that children could still be detained in the window after Christmas and before the formal ban.

Yet this is still a rare example of a genuine Lib Dem achievement and one for which Clegg deserves much credit. There is no doubt that the policy would not have been pursued by a Conservative-only government.

After this Lib Dem success, the scene is now set for a series of policy showdowns over control orders, banking reform, executive pay and an elected House of Lords. If Clegg is to live up to his boast that his party is pushing Tory ministers in a more liberal direction, he will need to prevail in several of these matters.

In an attempt to avoid the "fucking car crash" that David Cameron warned of, the coalition has again delayed a decision on the future of control orders. But the new year will soon pit Clegg against Theresa May and the security establishment.

It is a battle that he must not lose. On this occasion, the alibi of the deficit will not be available to him. Retention of control orders would amount to a fundamental breach of principle.