I have a small studio and have recently moved it from the UK to Spain. I have been using it in Spain for the last month or so with no problem. Today, when I plug in any of my three electric guitars into my Focusrite 6i6 interface I am getting a small, but noticeable and unpleasant electric shock when I first touch the strings or any other metal part of any of the guitars. I have had the three guitars for years, the Focusrite for ages too, I have not introduced anything new electrical into the house today. 

Can anyone suggest what could be causing this please?

I'm not familiar with the electrical service in Spain. In North America, I would get my outlet checker. It's a small device that you plug into the wall socket to test the polarity and condition of the ground (earth). It sounds like you may have an issue with grounding or possibly the hot and neutral have been reversed. This puts voltage on things that are usually grounded. In Canada or the USA, if you have an old amp with a 2 prong cord and plug it in, there's a 50% chance that power has been connected to the chassis and into the guitar via the ground circuit through the guitar cord.
 
Power utilities can mess up, mice eat wires and connections go bad over time. You need to have a good look, and be sure to wear rubber soled shoes until you get it sorted. Good luck.

John
Thanks for your reply. I'm kinda spooked by this and the whole area is a complete mystery to me. I think my only option is to seek a recommendation for an excellent English speaking electrician. Bill

Marshall
Today, when I plug in any of my three electric guitars into my Focusrite 6i6 interface I am getting a small, but noticeable and unpleasant electric shock when I first touch the strings or any other metal part of any of the guitars.

Me too. underside of the arm is more sensitive to it. In a land of mosquito, can have me thing I'm getting bitten.
 
I miss British electricians and the uk safety plug

Kamikaze

Marshall
Today, when I plug in any of my three electric guitars into my Focusrite 6i6 interface I am getting a small, but noticeable and unpleasant electric shock when I first touch the strings or any other metal part of any of the guitars.

Me too. underside of the arm is more sensitive to it. In a land of mosquito, can have me thing I'm getting bitten.
 
I miss British electricians and the uk safety plug

The underside of the arm on the bridge is the worst.

Are you using the same instrument cable for all of the guitars?  Have you tried a different one?
 
Could be the shield is shot on the cable, leaving no path for static drain.
 
Hey, it's worth a shot!  It would be a cheap fix

Yes this is static electricity seeking ground, The 6i6 uses a wall wart which isolates you from the A/C power. There is no ground. 
But the ground path will be from your guitars strings to the shielding in your patch cable, then it would somehow have to find it's way to your USB cable to the computer and then it can find the grounding via the computers power supply and A/C plug. 
 
I would see if it's your brand of socks or clothing causing the static. 
 
You could see if you get the same thing by touching a metal part of your computer case. 

I've had that happen too, and it's always been a grounding issue.

Had an electrician come round, changed the UK plug on my Uk four way adapter for a Spanish one. Previously, the UK plug was going into the mains via a travel adapter, and the electrician thought this was the weak link in the chain. It seems to have sorted it, but I'll be more confident in a month or so!

Thanks for your input.

I've had this issue for many years - and it's solved recently. Just another confirmation, it's always grounding one way or another.