Rome, September 12 - The interest rate on Italian bonds rose at two separate auctions on Thursday, with the rate on the three-year BTP reaching its highest level since October last year. The Treasury sold all four-billion euros' worth of the three-year bonds it put up for sale, but it had to give an average interest rate of 2.72%, up from 2.33% at an auction in July. The rate also climbed at an auction of 15-year BTPs, with 1.5 billion euros' worth going at an average rate of 4.88%, up from 4.67% at a sale in June. The Treasury also sold two billion euros worth of CCT bonds, which are index-linked to the eurozone's inflation rate. Italy's borrowing costs have been rising amid fears Premier Enrico Letta's fragile grand-coalition government may collapse in the fallout of last month's decision by the supreme court to uphold a tax-fraud conviction against centre-right leader Silvio Berlusconi.