Leaking sink?

Unscrew the nut, which looks cross-threaded, check any o-rings or seals for damage, replace any damaged parts, screw back making sure not to cross thread.

If the leak is actually from above the red line, that’s another matter.

 

If the water is coming from above the white nut ,marked in red in your pic, the waste fitting needs resealing.
If the leak is from the nut where it joins the thread of the waste fitting you need to replace the washer/ seal that sits between them

 

Usual cause of leaks from this area is (from my diy plumbing experience !) either;

Plastic nut too tight, causing rubber seal to deform

Plastic nut too loose, not creating a seal between rubber and flat of threaded waste

Damaged seal inside the nut / usually a fairly thin, flat rubber seal that can become damaged and partially disintegrate, leading to a leak

Rubber seal not seated correctly inside the plastic nut / twisted / etc, so doesn’t sit flat against threaded waste. As you pull the nut up to join to waste, it’s easy to dislodge the rubber seal from the neck of the trap

Seals are replaceable should it be damaged, enabling the bottle trap to be reused

Let the forum know the outcome / we like a follow up ! Good luck

 

There should be two plastic/rubber rings at the top of that bottle trap. One hard plastic, one softer rubber. I bet someone's got them the wrong way around (soft one next to basin outlet, gard one next to plastic threaded ring) or has otherwise mucked it up.

 

Unusual for there to be ‘two rubber/plastic rings at top of bottle trap’ - in fact, there won’t be !

Just a flat rubber seal inside white nut that seals against face of threaded basin waste

The ‘two rings’ set up will be where trap meets outlet pipe in wall -
So, white plastic nut / nylon thrust washer / tapered rubber seal

If I’m wrong, I’ll eat my hat (I’m a chef by trade so wear one on a daily basis) !

 

The one on the basin in my downstairs loo has one like that. It screws direct to the metal basin waste. Sounds as if it isn't a general thing (but it doesn't leak!).

 

You need to remove the trap from the waste fitting, then remove the waste fitting and re seal it. There should be a rubber seal between the large hexagonal nut and the underside of the basin.

 

If it’s a basin with an overflow, use a Basin Mate. It comes with the washer/seal you need for the underside too - https://www.screwfix.com/p/thomas-dudley-ltd-basin-waste-seal-kit-3-piece-set/8795R

 

If the leak is coming from the area around the white nut, try tightening the nut with a wrench. Sometimes, a loose connection can cause leaks. Be careful not to overtighten, as this could damage the fittings. Also, Check the seal or washer between the sink drain and the pipe. If it looks worn or damaged, you may need to replace it. You can find replacement seals or washers at your local hardware store. If the leak persists, you can try applying plumber's tape to the threads of the pipe before reattaching the fittings. This can help create a tighter seal and prevent leaks.

 

Looks like the waste fitting is askew, not fitted evenly.
Is there a larger gap at the front ,than at the rear, on the underside of the basin ?
If so ,try to refit the waste and influence it away from the wall,whilst tightening it up.