post graduate thesis

I am studying on an MSc at imperial. I am free the whole summer to do a thesis and I have to hand in before September. Could anyone advise me from experience whether it would be viable to do a 3 month internship over the summer as well as my thesis?

Imagine how much better your thesis will be if you spend an additional 3 months on it. How can you even seriously consider not doing that?

Because I feel like 3 months working as an environmental consultant would be better for me in the long run (I want to be a consultant and I currently have minimal experience). Also I doubt I could work on my thesis throughout the entire summer anyway sitting at home typing for the whole summer.

Well, I think it would be a shame to shorten the time you spend on your thesis, if you can also give it your best and try to get at least a publication and a distinction out of it. Maybe you have no ambitions to go into academia, but even then.

Sounds to me as though you're already certain that the value you'll get from the internship exceeds the potential extra marks you might forgo on the dissertation. If that's the case, you already have your answer! I'd just advise having a plan for progress on the dissertation and making sure you stick to it. I would caution however that as a rule the level expected of a master's dissertation is a step up from undergrad, so don't be too complacent about the 'minimal effort' you required then.

I am studying on an MSc at imperial. I am free the whole summer to do a thesis and I have to hand in before September. Could anyone advise me from experience whether it would be viable to do a 3 month internship over the summer as well as my thesis?