David Cameron's promise of an in-or-out referendum on Britain's EU membership has been dismissed as "ludicrous" by the Liberal Democrats.
Deputy prime minister Nick Clegg made clear yesterday he viewed the Conservative leader's plan to hold a referendum in the first half of the next parliament as "not in the national interest".
Now the party's backbench foreign affairs spokesperson, Martin Horwood, has gone further, warning that the prime minister's approach is "crazy".
"We don't know what's going to happen at the next general election, we don't know who's going to be in government," he told politics.co.uk.
"Clearly we don't know what coalition negotiations will take place and between whom. It is all rather crazy."
Cameron used his speech on Britain and Europe to unite his party behind a pledge to campaign in the 2015 general election for a mandate for the renegotiation and referendum.
His approach does not clash with the current coalition government's attitude, which is to hold a referendum only in the event of significant treaty change. Lib Dems have previously vociferously called for a vote on Europe, but are now seeking to stand apart from their coalition colleagues on the issue.
Horwood added: "I think it's a mistake to hype up the possibility of leaving at a time when we have a very serious economic situation. To have this idea of a hypothetical referendum four years in advance, and to raise that uncertainty, was very poor timing on David Cameron's part.
"If even a significant proportion of our potential foreign investors have been freaked to the extent they question whether to invest in jobs in this country, I think we will have done the people of Britain a grave disservice by raising this spectre."
The Lib Dems support many of the reforms Cameron called for in his renegotiation, but are suggesting this could be achieved without the "debilitating debate" which will dominate European politics over the next four years.