Four energy suppliers who owe nearly £ 15 million in green taxes risk losing their licenses.
Robin Hood Energy, Toto Energy, Gnergy and Delta Gas and Power were given a month by the industry regulator to make mandatory renewable energy payments after missing the original September 1 deadline.
Ofgem warned that suppliers must fulfill their obligations "or pay the consequences".
Within the industry, non-payment indicates that a company may be in trouble.
Of the three that missed the second deadline last year, URE Energy lost its license while Economy Energy and Spark went bankrupt.
Industry regulator Ofgem gave suppliers a month to pay the money owed
The money pays for renewable bond certificates, which demonstrate to Ofgem that companies are funding green initiatives in the move to move the UK from reliance on fossil fuels.
Robin Hood Energy, a not-for-profit provider owned by Nottingham City Council, owes £ 9.4 million.
Toto Energy owes £ 4.6 million, Gnergy 673.876 and Delta 91.937.
Mary Starks, Ofgem's executive director of consumers and markets, said: "Renewable obligation schemes provide important support to renewable electricity generators and play an important role in Britain's journey to a net zero emission economy. by 2050.
"Supplier failure to comply with the schemes compromises their integrity and is unacceptable.
"It also increases the costs of other suppliers who meet their obligations because they need to absorb or compensate for any deficits."
"This enforcement action sends a strong signal that suppliers must meet their obligations or pay the consequences, which could mean losing their license."
The Nottingham City Council has summoned problem solvers to try to reverse Robin Hood Energy.