Euro zone governments' drive to push back the expiry date of their debts stepped up a gear this week, as France sold 20 and 50-year bonds to lock in ECB-depressed borrowing costs. 

The European Central Bank's bond-buying programme has pushed short-term yields into negative territory and has brought longer-term yields closer to zero. 

The programme was expanded in March to €80 billion a month from €60 billion,

The top-rated countries in the euro zone have a choice between effectively being paid to borrow for up to five years or paying a meagre interest rate to borrow for 10 years or more. 

But extending the debt's average maturity allows a sovereign to reduce refinancing pressure and boost investor confidence in its ability to service its obligations. 

France yesterday priced a bond expiring in 2066, which is six years longer than its previous longest-dated bond, at only 18 basis points over the yield on its 2060 bond. 

Two weeks ago, Ireland issued its first 100-year debt at an yield of 2.35%, which was a little higher than it paid to borrow over three month in 2012 on its return to the market after a two-year hiatus at the height of the crisis. 

Last year, Belgium sold €50m in 100-year debt. 

Pension funds and insurers are the main investors in bonds with ultra-long maturities. 

They tend to buy and hold so they are attracted by the yield premium over shorter-term debt and are not put off by the risk of a sudden sell-off if the economic outlook improves and the ECB stops buying bonds. 

Japanese investors are traditional buyers of French bonds, attracted by the high ratings and yield pick-up over Germany. 

The Netherlands also sold €875m of 30-year bonds this week, while Germany sold €368m of 30-year inflation-linked bonds. 

Some investors will use the debt sales as an opportunity to switch from German debt into slightly higher-yielding Dutch and French bonds, which are also relatively safe. 

This called a halt on German 10-year Bund yields' push towards zero.