For Intra EU travel, this depends on the regulations within each EU state, for example for the UK you do not have to declare cash, when entering the UK from another EU state.
Of course, if you find yourself being stopped for a random customs check whilst entering the UK, and you are found with a substantial amount of cash, then you will have to answer some pretty valid questions in relation to Counter Terrorism and Money Laundering, but providing you have a valid reason there should be nothing to worry about. That said, money can be seized to allow further enquiries to be carried out.
This from the UK HMRC (Customs) Page:–
Travelling in the EU
If you bring cash to the UK from another EU country, you don’t need to declare it.
You may need to declare cash you take in to other EU countries – check with the authorities in the country you’re travelling to.
This advice from the EU seem’s contradictory, but relates to the fact that some countries, within the EU, may have separate rules on declaration.
“I recently had a large amount of cash confiscated by customs on arrival in another EU country, for failing to report it. I thought there were no limits on the amount of money I could carry around the EU”
You’re right in principle – although you are obliged to declare large amounts of cash (or equivalent, such as travellers’ cheques, promissory notes, monetary orders) to customs.
EU law still allows individual EU countries to have national cash control measures, provided they’re not excessive, including obligatory declaration.
One of the main reasons is the public interest, especially public security – to combat money laundering and the financing of terrorism.
Remember also that when leaving or entering the EU, you must declare all cash (or equivalent) worth €10 000 or more.