What kind of plant is this? Can you photo the entire plant? It looks like the work of a leaf miner. A bug of some sort is chomping through the plant's vascular system. The way to control it is to snip off any infected leaves before it spreads. The bug is inside the plant so a systemic insecticide may be necessary.

The damage is from one of the many varieties of leaf miners. Bathgate has advised you on its control/ eradication.

If it is a perpetual spinach or a Swiss chard, then it is definitely a leaf miner. It is a very serious problem here in London and a very difficult to control. Myself and many of my friends stopped growing these vegetables. Just spray with a systemic insecticide if you want to keep it under control.