A British Challenger 2 tank has been badly damaged by a roadside bomb in Iraq, leaving the driver seriously injured, the Ministry of Defence said.
A spokesman said the tank had been on routine duties in a western district of Basra when it was hit on 6 April.
It is believed to be the first incident where a Challenger tank has been breached by such an attack.
The driver of the Challenger has now been sent back to the UK for further medical treatment.
An MoD spokesman said the Challenger tank was "well armoured but in an operational theatre it's not the case that you can have absolute protection".
He said: "This was not in any way new technology - the device involved was the same type of shaped charge that we have seen used very regularly.
"No-one has ever said Challenger tanks are invincible.
"We have always said that a big enough bomb will defeat any armour and any vehicle, and the Americans have lost many tanks in Baghdad."
He added that the attack also left one of the other three tank crew with minor injuries.
BBC defence correspondent Paul Adams said the Challenger was generally used on the ground only in situations where the Army felt the need to make "a big statement".
The Challenger 2 is the UK's main battle tank, and its key function is to destroy enemy tanks.
It has a good reputation for reliability, although it has experienced significant problems during military exercises in desert conditions.
The UK has almost 400 Challenger 2s, some of which have been used in Bosnia and Kosovo and have a four-man crew.
In addition to its Advanced Armour Technology, the Challenger 2 also has a nuclear, chemical- and biological attack-resistant compartment for the crew.
Professor Michael Clarke, from King's College's Defence Studies centre, said the Challenger 2 tank's armour was usually "inviolable".
He said: "Most of the things on a battlefield are not much of a threat to a tank, usually.
"This is worrying, because if there are many of these sorts of very heavy penetrative improvised explosive devices around in the area then no vehicle is safe."