Black soot (?) on the back of gas boiler

Not sure if this is the correct place to post this. But on our gas boiler, if you run your finger along the back of it, its covered in a black soot-like substance. Is this a problem? I was surprised to find it there, I would have assumed it would be clean or dusty at worse.

The outflow from our boiler goes out through the side wall of the house to the outside.

It is very important that you have the boiler checked out urgently by a qualified person,soot on a gas boiler is dangerous!

Thanks John. I'll get someone in over the weekend. I'm a bit worried now.
Isnt there some gadget you can buy to stick beside boiler that checks for CO output? Do you know what its called?

You can get carbon monoxide detectors in the larger DIY shops.
Leo

Nothing wrong with being careful, but my first guess would be burnt dust rather than soot.

Make sure when buying a CO alarm that it is a proper 'alarm' not
a creditcard type with the dot that blackens.

Soot is a visable sign of 'incomplete combustion' which is a positive
indicator of carbon monoxide, turn off the appliance and do not use
until it has been correctly checked and serviced, also ask your serviceman
if he can perform a carbon monoxide room safety test (takes about 30mins)