By Ben Brown BBC correspondent, with British forces in southern Iraq

So it is now official. Hundreds of British troops are to be moved to central Iraq to relieve US forces there. The first battalion of the Black Watch - about 850 soldiers in all - are set to move north from Basra to the Babil region close to Baghdad.

Black Watch have been told they will be home by Christmas

Commanders here in Basra say it's not easy to move so many troops with so much equipment so far north.

That is why detailed and logistical planning for this controversial deployment has been going on for several days, well before the official announcement in the House of Commons.

At their base the Black Watch have been getting ready to go. After all the controversy and uncertainty, they know at last they are heading north.

Difficult job

Parts of their preparation has included adding extra protection to their warrior armoured fighting vehicles. They know that where they are going Iraqis may be a lot less friendly then they have been in and around Basra.

"It's going to be a difficult job. There's no doubt about that," said British Army spokesman Major Charles Mayo.

BRITISH REDEPLOYMENT 500 Black Watch soldiers with up to 50 Warrior armoured vehicles 100 Royal Dragoon Guards with 12 Scimitar armoured vehicles 50 Royal Marines light infantry from 40 Commando 300 support personnel such as engineers, logisticians, signallers and medics Hoon statement in full Profile: The Black Watch

"But this is an experienced battle group. They were here last year during the war. They have had some interesting times in the last four months . They're an experienced bunch of guys and they have a lot to offer."

"And it's a relief for them. They have been waiting for a decision. They've now got it and just want to get up there and get on with the job."

But getting up there will be no mean feat. On the ground and in the air, the British will be carrying out surveillance to make sure the route north for the Black Watch is as safe as possible. The journey will take them hundreds of miles through potentially hostile terrain.

Moving in convoys, there are fears insurgents could be waiting for the Black Watch with ambushes and roadside bombs. Because this deployment has been so widely discussed, it has also been widely advertised.

Promise

And that is why no-one here is talking about when they will be leaving. After a very public debate in Britain, commanders here now want to maintain as much operational secrecy as possible.

The Black Watch are well aware it will be a dangerous few weeks for them but at least they have a promise that they will be home by Christmas.

Their commanders will not want to break that promise. But that could mean other British troops would take their place if US forces have to spend more time on operations in and around Fallujah.

For the British army, it opens a new chapter in a new part of Iraq. Their critics have called it mission creep.