price war 
noun
A period of fierce competition in which traders cut prices in an attempt to increase their share of the market: small garages could be forced out of business in the petrol price war...
- At the same time, though, he's raising fears within the industry of a price war.
- Supermarkets and filling stations launched the latest battle in the petrol price war yesterday with a new round of cost cuts at the pumps.
- A good few years back two of the big supermarkets went head to head on these items in a fierce price war.