The euro fell to $1.2158 overnight, its lowest since September, but rebounded slightly to $1.2261 by midday.

The second "No" vote in a week has prompted fears that economic reform and integration across Europe will slow.

But despite such concerns, the European Central Bank (ECB) voted to leave rates unchanged - meaning they have now been stationary at 2% for two full years.

The zone's "one size fits all" rate has been widely criticised as too high for some struggling economies but not high enough to rein in inflation and potential asset bubbles elsewhere.

But commentators have said that while the "No" votes from France and the Netherlands have not helped, the main reason for the currency's current lows was the recent string of interest rate rises in the US.

'Business as usual'

Meanwhile, Dutch central bank chief Nout Wellink dismissed claims that European Monetary Union could break apart as a result of the vote.

"It's utter nonsense. For us, it's business as usual," he said as he headed to the ECB rates meeting.

I think the big question is the big foreign investors... if their perception of the Netherlands changes and therefore money fails to come in Ron Van der Kroll, Het Financieele Dagblad

The euro is still well above its launch value against the US dollar, and its recent decline is expected to boost exporters in the eurozone.

Analysts said that hostility to the euro, the sluggish European economy, fears of a European super-state and concerns about immigration had prompted 62% of Dutch voters to reject the constitution.

Ron Van der Kroll, of Dutch newspaper Het Financieele Dagblad, said the poll would have no "immediate effect" on the country.

But he told the BBC: "I think the big question is the big foreign investors, American or Asian companies.

"If their perception of the country changes and therefore money fails to come in, the Dutch economy will certainly feel it."

Euro 'anger'

Mr Van der Kroll added that the vote was an "emotional" reaction from the Dutch people who were angry at big price rises since the euro's introduction.

The Dutch population vented their anger over the euro in the vote

Experts said the vote also casts doubt on the EU's plans to expand further into Eastern Europe.

European Commission President Jose Manuel Barroso acknowledged that the Dutch vote meant difficult days ahead for the EU.

"I think we have a problem, a serious problem," he said.

Observers said the constitution was dead in the water.