This treaty is being negotiated in secret between the EU and USA and amounts to a complete corporate takeover and an end to effective democracy. This link is the best explanation of the problems with it that I have found so far:
I note that 38 Degrees have also started a campaign against it. See: https://secure.38degrees.org.uk/ttip-leaflet-action.
Of the UK political parties, this promised disaster is supported by Labour, Tories, LibDems and UKIP, and opposed only by the Green Party and the Scottish and Welsh nationalists. Mussolini would have loved it.
While I would be reluctant to see us leave the EU (although it needs a lot of reform), if this goes ahead I would regard it as sufficient reason in itself to do so.