Nine European Union countries, including Greece, face potential legal action for failing to implement new rules on pig welfare, the European Commission said on Thursday.

A quarter of the bloc's 12 million sows are still being kept on farms where individual cages, known as sow stalls, remain in use, even though they were banned from Jan. 1, 2013, EU sources said last month.

Final written warnings were sent to Poland, Denmark, Greece, Belgium, Portugal, Ireland and Cyprus as well as Germany and France. If the nine countries fail to respond adequately within two months, the Commission said it would start formal legal proceedings.

"Despite repeated calls by the Commission, (these) ... member states have failed to adequately comply with EU law," the EU's executive said in a statement.

There are fears some producers will not be able to afford the investment in new stalls and be forced to close, which could reduce pork production, increase prices for European consumers and dent soaring EU exports to Russia and China.

Sow stalls, or gestation crates, are metal cages roughly 2 metres (6-1/2 feet) long and 60 cm (2 feet) wide used to hem in pregnant pigs and make them easier to control.

Britain outlawed sow stalls in 1999, while several other countries, including Sweden and Luxembourg, banned them before the EU deadline. [Reuters]