At the moment, there's several sites around the internet which claim that modern British tanks, such as the Challenger 2 use Explosive Reactive Armour. I believe this is wrong, and that the Panels used are solid blocks of Composite armour. The only conclusive evidence I've seen for ERA are the various websites, all seemingly quoting each other, saying its ERA. The only reason I can see for this is that the blocks look sort of like ERA panels. Now there's the obvious issue that this is current or semi-current equipment so it's all wrapped up in operational security issues. So this argument against ERA is based on purely publicly available sources.

Within NATO ERA is classified as an ammunition type which when you think about it makes sense. Therefore you have to provide "re-loads". Equally it obviously contains explosive, so you need to deal with it like you would any large amount of explosive. So with that in mind, let's take a look at the arguments against the upgrade armour being ERA.

First off, as is often the case when dealing with governments, it's best to follow the money. This link is the National Audit Office's report into Operation Telic, which was the codename for the UK's part in the 2003 invasion of Iraq.

Page 19 states:
"In October 2002, the Department approved a separate Urgent Operational Requirement to fit a new generation of appliqué armour to 137 Challenger 2 tanks, of which 116 vehicles were deployed to theatre. The total cost of this package was £8.8 million"
The important part is in bold. "Appliqué armour" indicates it is an inert set for each tank. You'll also note the utter lack of mention of reloads. Just the one set for each tank. That would indicate to me that it's not ERA.

Next we look at the one Challenger 2 destroyed during Operation Telic:
A modern armour expert I know points two things out here. ERA doesn't explode when set on fire, instead the explosive provides added fuel for the fire, which means there would be increased scorching. This increase of burning appears to be absent. He also points out that the blocks look too thick for ERA.

Finally we have the safety aspect. ERA is dangerous to those squishy human things around the tank when it goes off. So in consequence you don't want to operate it around or near civilians after a shooting war is over, otherwise you might blow some of them up, which sort of wrecks the hearts and minds approach.
Here we see Challenger 2's in very close proximity to civilians in Iraq, all with the armour packages fitted:

But you might argue that depending on the threat state the tanks would take the risk, putting protection of the tank and their crews over the safety concern. Fair enough. What about Europe?
However, I will admit the location could be anywhere, maybe Germany, or maybe Kosovo. So the protection might be required. But as a final clinching proof, here's a picture of a Challenger 2 driving around Bovington tank museum arena:
Now I'm qualified in Health and Safety, and on a professional level I'd love to read a risk assessment for driving several pounds of explosive around in a small crowded area with several thousand members of the public...

As a supporting point, here is a picture of a Warrior that took an RPG hit to the side:

Now there's only a few mm of armour underneath that panel. Yet the RPG didn't penetrate, and the panel obviously didn't explode. Can anyone find a picture of a Challenger 2 with a panel that has exploded? Or any data to support the claim it is ERA?
Its also worth bearing in mind that the original NATO briefing (Would you like to know more?) on ERA mentioned Burlington as small blocks that could easily be replaced, or mounted on existing tanks and AFV's.
Two Slides taken from the original Burlington presentation.
Finally, I've never ever seen a picture of a AFV so fitted, with a panel that has exploded. Can you find one?
So for all the reasons above I think that most of the internet is wrong, the add on armour packs are not ERA, but solid lumps of composite armour.

Image credits: