Iain Duncan Smith, one of the cabinet's most eurosceptic members, caused some excitement at the weekend when he suggested that a referendum could be held on any "major treaty change" (such as that required for a fiscal union) even if there was no further transfer of power from Britain to the European Union.

"If there is a major treaty change, it is now legislated for that we should have a referendum," he told Sky News, adding that this was David Cameron's position. But the Work and Pensions Secretary has now been slapped down by Downing Street, which made it clear that the coalition's "referendum lock" only applies to treaties that affect British sovereignty.

The prime minister's official spokesman said:

What is being discussed at the moment is about how the members of the eurozone organise themselves and how they construct the right economic governance for the eurozone ... There are no proposals on the table for a transfer of powers from the UK to Brussels. That is not what it being talked about ... There is no lack of clarity here. The Act talks about triggering a referendum if there's a transfer of power from London to Brussels.

With reference to Cameron's previous "cast-iron pledge" to hold a referendum on the Lisbon Treaty, ConservativeHome's Tim Montgomerie suggests that Downing Street's position is "technically correct" but that voters will see it as "slippery". With this in mind, it's worth noting that the PM's spokesman refused to explicitly rule out holding a referendum on a major treaty change that didn't affect Britain. As Andrew Sparrow notes, "The Act specificies the circumstances in which a referendum has to be held (a transfer of power), but it does not stop a government holding a referendum in other circumstances."

As Germany and France pursue "ever closer union", expect this debate to run and run.