European Commission President Jose Manuel Barroso said states breaking away from existing European Union countries would struggle to gain EU membership, further complicating Scottish nationalists' already uncertain plans for independence.
Barroso said in an interview it would be nearly impossible for the EU to grant membership to such states - days after the British government said an independent Scotland would not be able to keep sterling as its currency.
Scotland is due to hold a referendum on independence in September. Polls show around 29 per cent of voters in favour and 42 per cent against, with 29 per cent undecided.
Barroso, interviewed on BBC television yesterday, declined to comment directly on whether an independent Scotland would be welcome to join the EU.
But he said all EU states would need to back the membership of any new country that emerged from a current member state.
"It would be extremely difficult to get approval of all the other member states ... I believe it's going to be extremely difficult, if not impossible," he said.
The Scottish National Party, which is fronting the independence campaign, is banking on retaining both EU membership and the pound.
John Swinney, an SNP deputy in Scotland's parliament, told the BBC Barroso's comments were "preposterous" and that no EU state had indicated it would veto Scottish membership. But secession is a sensitive subject for several other countries that have regions seeking to form their own states.
Spain, which Barroso said in the interview had been "opposing even the recognition of [former Serbian province] Kosovo", is for instance wary that a vote for Scottish independence might encourage separatists in Catalonia.
Scotland's First Minister, Alex Salmond, also went on the offensive yesterday against critics of the independence campaign.
Writing in The Sunday Times, he accused the British government of bullying over the currency issue and said he had asked Prime Minister David Cameron to rein in his campaign to keep Scotland's 307-year union with the rest of Britain intact.
But the leader of the campaign to keep Scotland in the UK, former British finance minister Alistair Darling, said the independence drive was unravelling.
"Alex Salmond is a man without a plan on currency and Europe. The wheels are falling off the independence wagon," Darling said.