The company scored just 0.35 stars out of a possible five for its customer service between January and March this year.
The firm received more than 9,000 complaints per 100,000 customers, five times higher than the next worst supplier, TOTO Energy, which received 1,800 complaints.
So Energy placed at the top of the satisfaction rankings with a score of 4.8 stars, while SSE took top slot among the Big Six with a score of 4.25 stars out of five.
Iresa has been unable to take on new customers since March due to a provisional ruling from the energy regulator Ofgem.
Citizens Advice believes it is currently too easy for firms to set themselves up as a new energy supplier and suggests the regulator should tighten its licensing rules.
Gillian Guy, CEO of Citizens Advice, said: “Iresa’s low score will come as no surprise for its customers, who have had to put up with continued problems.
“These include inaccurate billing, long phone queues and even being blocked from switching to a new supplier.”
ELN has contacted Iresa for a statement.