"[We] passed a law to guarantee a referendum before Britain passes any more powers to the EU [and will ensure that] any referendum triggered by the EU Act is on the big question: In or Out"—Liberal Democrat manifesto
"Labour will legislate for a lock that guarantees that there can be no transfer of powers from Britain to the European Union without the consent of the British public through an in/out referendum"—Labour manifesto
To the casual reader, Labour appears to be saying that it will pass a law that's already, according to the Liberal Democrats, in force.
The Liberal Democrats are referring to the European Union Act 2011. This requires the support of a majority vote at a referendum before the UK ratifies any treaty that would transfer further powers to the EU. But it doesn't give rise to a referendum on whether the UK should leave the EU.
It appears from these manifesto pledges that both parties intend to strengthen the European Union Act 2011 so that the referendum triggered by a transfer of powers is on whether or not to leave the EU.
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