Britain and EU strike deal on nations blocking unwanted new laws

By William James LONDON (Reuters) - Britain has reached an agreement with European Union officials on a mechanism to block unwanted EU legislation, a source in Prime Minister David Cameron's office said on Tuesday, in a deal that meets one of Cameron's four main reform targets. European Council President Donald Tusk later on Tuesday will publish a draft proposal for reforms to the EU designed to satisfy a series of demands made by Britain ahead of a planned referendum on the country's membership