With the ink still drying on the €85 billion European Union/International Monetary Fund deal to save Ireland, warnings have been raised about Belgium because its debt is equivalent to 100 per cent of its GDP.
A recent austerity package was not seen as strong enough and financial speculators are said to be eyeing the value of its bonds.
The Iberian countries of Spain and Portugal had been most analysts' favourites to be next to seek financial aid, however Belgium's crisis-hit political system and threats of separation by the Flemish part of the country have made it even more vulnerable.
The divisions have been made worse by the two groups' views on how to deal with the economy.
The majority of Flems want austerity measures similar to the UK, but the more socialist-inclined Walloons have managed to block major government cuts.
In the past, comparisons have been drawn with a push for separatism for Scotland, but unlike the UK the split in Belgium has destabilised the country's government.
In April the government led by Yves Leterne collapsed when it failed to find a solution to the economic crisis and national divisions and since then the premium to insure Belgium's debts have risen by 5 per cent.
Since then the country has been without political leadership, as rival parties attempt to form a ruling coalition.
• Ireland: Sinn Fein set to cash in on bail-out discontent
• Portugal: Debts pressure pushes up loan costs
The political stalemate has left Belgium with a caretaker government and no budget for 2011 more than five months after inconclusive elections.
Yesterday the caretaker Belgian government denied that it was in trouble, arguing that, unlike Greece, Ireland and Portugal, most of its debt is owned by Belgians. The Belgian government also said it was in a better position than the UK.
A government spokesman said: "This makes the character of our debts different to the UK. We are net savers, the UK has borrowed more internationally."But he added: "It's unfortunate we must wait to form a new government, but it's a democratic process and we'll resolve it."
Yesterday, German chancellor Angela Merkel tried to restore confidence in the eurozone, which her country has been underpinning since the Greek crisis and €110bn bail-out.
Germans are thought to be getting nervous about the way they are being asked to pay up loans for weak European economies because they are all part of the same currency.
"I'm more confident than this spring that the European Union will emerge strengthened from the current challenges," Ms Merkel told business leaders in Berlin.
However, there were concerns over the financial situation in Spain, which was last night said to be teetering on the brink and would be the biggest economy so far to be laid low by the crisis.
Experts say that while rescuing Greece, Ireland or even Portugal is manageable for the EU's €750bn emergency fund, bailing out Spain - whose economy is five times larger than any of the other three countries - would test its limits and threaten the euro's very existence.
Axel Weber, the head of Germany's central bank and a leading rate-setter at the European Central Bank, said European nations would be willing to boost the emergency fund by as much as €100bn to fully cover the total public debt load of Greece, Ireland, Portugal and Spain.
The leaders of Germany and France, the eurozone's twin economic engines, were discussing the European debt crisis in a telephone call later Thursday.
In the markets, investors continued to put pressure on Portugal and Spain, keeping their borrowing costs near euro-record highs.
That reflects market uncertainty about the countries' ability to pay off debts amid a downturn - and fears that they will also need massive bail-outs.
Markets demand a higher return on bonds issued by countries seen as a risky investment.
"Uncertainty has got a firm grip on the market, that much is clear," said Filipe Silva, a debt manager at Portugal's Banco Carregosa.
"Comments by (European leaders) aren't giving the market any sense of direction."
Nomura strategist Alastair Newton yesterday warned several vulnerable economies "could just take the eurozone to the brink again".