The yield on 10-year Portuguese bonds hit 17.38% this evening, the highest level since the euro began.
The rise came as investors worried that Portugal might seek a second European bail-out, following in Greece's footsteps.
Earlier, Italy sold €2 billion in 10-year bonds at sharply lower rates in a series of auctions that raised a total of €7.5 billion - close to the maximum target amount.
The rate on 10-year bonds fell to 6.08% from 6.98% in December, the Bank of Italy said, a sign of improved investor confidence in the reform-minded government of former economics professor Mario Monti.
The auction, which comes as EU leaders hold a summit in Brussels on the debt crisis, also brought in €3.57 billion via a sale of five-year bonds at a lower rate of 5.39%. It also raised €746m in bonds due in 2016 and €1.16 billion in bonds due in 2021 at rates of 4.79% and 5.74% respectively.
Total bids came to €10.6 billion and the government had been hoping to raise between €5.5 billion and €8 billion.
Euro zone market tensions have eased in recent months but remain elevated due to uncertainty over talks between the Greek government and private creditors over a massive sovereign debt write-down.
ECB bond buys fell sharply last week
Data published by the European Central Bank show that the bank's purchases of the bonds of euro zone countries diminished sharply last week amid signs of stabilisation on sovereign bond markets.
The ECB bought just €63m of bonds from euro zone countries on the secondary market, compared with just over €2.24 billion the week before. As usual, the ECB did not specify which country's bonds it had bought.
The bank has now bought a total €219 billion in euro zone government bonds since it first began the controversial operations in 2010 as part of efforts to ease debt strains in the zone.
It resumed major purchases in August when renewed strains pushed Italian and Spanish borrowing rates to unsustainable levels but purchases have dropped sharply in the past two weeks.
Several European governments have put pressure on the ECB to step up its programme of buying bonds, seen by some as a possible solution to the debt crisis. But ECB President Mario Draghi has insisted that the programme is only temporary.
