There might be a way to disable this, but if Wi-Fi is switched on and you find it useful when it finds a network you can actually go on and transfers the internet it can be pretty useful, however if all it finds is some network that doesn't let you on and yet opts out of the 3G signal for that it is rather annoying. Eg I went to a chippy near a library and found I had no internet because the phone had logged on to "Libraries Wi-Fi" which gave no access because you had to register first, couldn't the phone have a way of checking that there is internet access before committing to a Wi-Fi network and turning off internet via 3G?

My Android phones (Galaxy S, now Nexus 5) have never done this.

I tend to use an app called WiFi Manager in conjunction with an app called Smart WiFi Toggler to manage my wireless connection as I have two access points in my house and I like it to switch to the stronger connection automatically as I move from downstairs to upstairs. When there is no signal, my wireless automatically toggles off and comes back on only when I am back in range again (e.g. returning from work).

Any other wireless networks I manually connect to will be added as a remembered network and adopts the same behaviour. I don't think either phone has automatically connected to a network without my intervention nor would I want them to.

If you say which phone you have maybe someone can advise how to set it up so it doesn't behave in the manner you describe.

Some android phones have an option to check for Internet connectivity when joining a wifi network

Yes, in the Advanced options (in 4.4 but probably other recent versions) there's an "Avoid poor connections" option which infers it'll sidestep such connections (in that it states "Don't use a WiFi network unless it has a good internet connection").

iOS does this already - if the WiFi signal doesn't let the phone check the apple website then it pops up an overlay browser window so you can enter login details. If you cancel that, it gives up the connection and goes back to 2G/3G/4G.

And what happens if you leave your phone in your pocket and don't check the screen?

I don't think it just connects to random wifi networks automatically anyway, unless you've connected to it before.

But the OP is specifically asking about setting it up so you do autoconnect to every random network it sees.

Oh yeah. Well, the OP hasn't been back to say what phone/platform he's using, so I suppose it's anyone's guess at the moment.

I had this problem with the cloud (which I needed at the gym but did not want anywhere else) - Android 4.3 (and 4.2?) has it built-in and on by default - Settings / WiFi /advanced options / avoid poor connections.

If you're using an older android, there are plenty of apps available.