"As in all areas of the market for sustainable finance, we are wary of issues that offer less transparency in terms of use of proceeds, or whose structures are likely to raise questions,” portfolio lead at NN IP Bram Bos acknowledged.

Past issuers have either entered the market with bullet issues, the case in France and Belgium, or chosen to build a yield curve by issuing green bonds with different maturities and coupons, where Poland is an example. These are the preferred structures. A green yield curve signals that a country is committed to a fully-fledged green bond market, Bos said. Bullet bonds only pay out at maturity.

Although the global market for green bonds is marginally down for the first half of 2020, it has doubled since 2019, according to figures from S&P Global. UBS also sees the market firing up. On yesterday's call, its bond specialist said that green issuances in June reached $17.8 billion versus $8.9 billion in social bonds, but the market for social bonds is up more than 250 per cent year on. Samantha Sutcliffe, head of green bonds for the Swiss manager, said the newly launched ICMA principles have helped provide "that extra comfort for issuers to come to market." She said that banks and corporates were showing far more interest.