Pop-up window for Technicolor Gateway password

[#1]

At seeming random times, we get messages popping up on both the 2 computers something like:
"Technicolor TG582n Plusnet, Login, Enter your username and password to access your Technicolor Gateway"
We just cancel it and carry on as normal.
I assume this is something to do with the router, supplied by Plusnet. What on Earth is it babbling about? How do I get rid of it or it is some amazing thing to get to untold broadband warp speed or whatever?
Any ideas? No jargon, please, I'm not a techie. Thanks.

[#2]

If you entered 192.168.1.254 into the address bar of your web browser and pressed the enter key then you could summon up the pop-up window you refer to.  But I do not know of anything that could cause the pop-up to appear unbidden on two different computers so I can only presume it is a fault on your particular router. 

[#3]

Hi,
When this message pops up on both computers, have you noticed if the Internet Light is on at all?
If this continues to happen, I would advise clearing out your Cache and Cookies. You can find out how to do this here: http://www.wikihow.com/Clear-Your-Browser's-Cache - You'll just need to choose the browser you are using from the list.
Let me know if this helps at all

[#4]

I suspect it's a combination of two things causing the problem.
Like Chris has mentioned, I think your Internet connection might be dropping out. When it does the router is using it's 'web browsing interception' feature to try and push you to the router settings pages.
You can disable this feature (see attached) but if we're barking up the right tree then instead of the login screen, you'll get a web page time-out instead.

[#5]

I've experienced this.
And it's what they said ^^^ the connection dies and the browser defaults to the router settings pages/login.