According to the official Glossary on the Europa website
The Community Charter of the Fundamental Social Rights of Workers was adopted in 1989 by all Member States except the United Kingdom. The objectives of the Charter have been included in the Treaty of Amsterdam since the integration of the provisions of the Maastricht social protocol in the Treaty. The Lisbon Treaty makes reference to it in title X on social policy (Article 151 of the Treaty on the Functioning of the European Union).I seem to recall arguing this for some time with a number of people who "knew better".
It so happens that the subject came up in two Written Questions in the House of Commons:
Mr Douglas Alexander: To ask the Secretary of State for Business, Innovation and Skills what recent assessment he has made of the costs and benefits of proposals to withdraw from the European Social Chapter; and if he will place any such assessment in the Library.Can we now stop talking about it as something we shall repatriate just as soon as that commission starts working on what it is we want to repatriate and how we should go about it?
Mr Davey: None. There is no distinct “European Social Chapter”; the EU's provisions for social and related matters are integrated into the treaties.
Mr Douglas Alexander: To ask the Secretary of State for Business, Innovation and Skills what representations he has received since May 2010 calling for the UK to withdraw from the European Social Chapter; and if he will place a copy of any such representations in the Library.
Mr Davey: There is no distinct “European Social Chapter”; the EU's provisions for social and related matters are integrated into the treaties. This Department regularly receives representations from stakeholders on matters concerning aspects of European employment law. It would not be practical to deposit all such representations in the Libraries of the House.