The interest rate Portugal must pay to borrow funds hit a fresh high on Monday, as speculation mounts that it will join Greece and the Irish Republic in needing an international bail-out.
The yield on 10-year Portuguese government bonds rose on Monday for the fourth consecutive day, to 7.16%.
Borrowing costs have surged due to the worries over the health of the economy.
However, Portugal has continued to maintain it does not need rescuing.
Lisbon argues its situation is different from Greece and the Republic of Ireland - both of which have agreed to bail-outs from the European Union and International Monetary Fund.
It says that its deficit and debt are lower than those nations, that it has not suffered a bubble in property prices and that its banks are sound.
The European Commission also said that there were no discussions underway on an EU-International Monetary Fund bail-out of Portugal.
An auction of government debt on Wednesday aims to raise 1.25 billion euros.
The auction will be closely watched as an indication of investor confidence, as markets look to see how easily - or otherwise - the debt-stricken nation can raise funds.
Bail-out fund being stretched
The rise in the bond yield came after Der Spiegel reported that both France and Germany were putting pressure on Portugal to access European rescue funds, in an effort to stop the crisis from spreading to Spain.
Analysts believe that while Europe could support Portugal, a bail-out of Spain would stretch the existing bailout fund.
However, a German government spokesman said that Germany was "not pressuring anyone, and has not pressured anyone in the past". The French government declined to comment.
Greece was the first eurozone nation to take a bail-out when a 110 billion euro was agreed, over three years.
An bail-out package of 85 billion euros for the Irish Republic was agreed last month.