According to Fortune Magazine, 93% of people work on a team where at least one co-worker isn't pulling their weight. Despite how frustrating the situation can be, only one in ten is willing to take any action. The truth is, if you have a colleague who does little of the heavy lifting, then you have to do something about it. You can't be expected to pick up the slack when their behaviour affects the whole team's performance.
Whatever you do, don't avoid the situation because it won't resolve itself. The first thing you should do is address them directly – be courteous and calm at all times, but let them know that they don't seem to put in as much effort and commitment as the rest of the team.
If they handle it badly or don't change their behaviour, take it to your supervisor and have a quiet word. You need to stop covering for them and let more senior figures know that there may be a problem. It's not telling tales or being underhand, it's protecting everyone else who might be impacted by the situation.
You need to make sure that their attitude and laziness don't affect your own performance. If you're dedicating too much time to doing their work as well as your own, standards will inevitably start to slip. Don't let them distract you or damage your own work ethic, otherwise you've handed them a victory.
Finally, avoid office gossip. When a coworker isn't pulling their weight, you have to act like the more mature person and resolve it cleanly.
You have to address the situation
Speak to them directly
Raise your concerns with your supervisor
Don't let them affect your performance
Don't stoop to office gossip