The dominance of Britain's four biggest supermarket chains is being challenged.

The Office of Fair Trading has been asked to investigate recent takeover acquisitions by Tesco, Asda, Sainsbury's and Morrisons.

It follows calls from Friends of the Earth, the Association of Convenience Stores and farmers' groups that the competition watchdog should carry out a full market review of the big chains.

The campaign comes after Sainsbury's recent takeover of 114 Jacksons stores.

Sandra Bell, of Friends of the Earth, said: "The domination of the grocery market by the biggest supermarkets has been allowed to increase, unchecked, since the last investigation more than four years ago.

"The Office of Fair Trading has so far turned a deaf ear on concerns about loss of small shops, bullying of suppliers and erosion of consumer choice."