The Daily Telegraph said a draft was to be released Tuesday.
"The Conservative Party will publish a draft Bill to legislate for an in/out referendum by the end of 2017," a source in the party told the newspaper. "We will examine all opportunities to bring this Bill before Parliament including as a private member's Bill."
Prime Minister David Cameron, who has promised to renegotiate Britain's relationship with the EU and then give the voters a chance to decide on membership if he is re-elected in 2015, was in Washington, where he met with President Obama. The president supported his plan, saying the prime minister should get the time he needs to "fix" the EU, the Telegraph said.
The Liberal Democrats, the Conservatives' junior partner in the coalition government, are not expected to support a bill mandating a referendum, which would prevent Cameron from introducing it himself.