It is the beginning of the end of that x member.
Weld it for now, but if keeping car budget for a new one in the near future, make sure you fit a glaved one with extensions.
Cut out the rot and flush weld a new section, take the opportunity to have a look down the chassis rail and clear out any crud. You could have a small hole in the new plate and squirt some oil diesel mix down the rail.
@lynall is right .. "It is the beginning of the end of that x member."
Everyone else is right too ... 'Cut out the rot and weld in a new bit' 2mm steel plate works. It was that thick originally.
Land rover chassis, X-members and outriggers rot from the inside there are also many other areas where mud and grime get trapped and collect water.
Cleaning regularly underneath and during those hot spells we get, when you know the chassis is dry, spray inside the chassis with a rust convertor (Dinitrol for example) and then with Waxoil or Lanoguard (or similar). The problem is that it may have never been done and after a number of years the rot has set in. Yours looks to be in that state - when did you last clean off and coat your chassis with an inhibitor ?
Start the engine, visually check for obstacles. Push the leftmost pedal (of the three) to the floor, move that waggly stick (in the middle) to the left and forward. Hold the steering wheel with both hands and (heres the difficult bit) press the right pedal gently down about 1/3rd whilst slowly releasing the left pedal .... If you are not in a field then you need to do other things like indicate, look in the mirrors etc.
Above all don't panic, you'll get the hang of it.
P.S,
If you fail to achieve forward motion, you may have left the handbrake on