"This has impacted Gentrack's sales pipeline to a greater degree than previously expected," it said.
Gentrack said it was in transition from an upfront license model to a recurring model, involving more predictable contracted payments.
A detailed full year 2020 "reforecasting" exercise had been completed in which forecast revenue had decreased significantly.
Gentrack now expects its full year 2020 earnings before interest, tax, depreciation and amortisation (EBITDA) to be between $8m and $12m, about half the previous year's EBITDA of $24.8m.
Gentrack said it was taking action to reduce its cost base by about $8m on a full year basis.
The company continued to be a profitable and cash generative business, with a strong balance sheet and no net debt.
Gentrack last year reported a net loss of $3.3m, which was largely the result of impairment charges.
Chief executive Ian Black told BusinessDesk no decision had been made about whether the company would be paying dividends this year.
Gentrack paid total dividends of 8 cents per share in the 2019 financial year, down from 13.7 cents a year earlier.
The software company aims to pay at least 70 per cent of adjusted net profit, depending on its outlook, and capital and liquidity needs.
Matt Goodson, managing director of Salt Funds, said the earnings update was worse than expectations, even after last week's update.
"The UK headwinds have been known about for some time but are more broad-based than earlier thought," Goodson said.
"Australia is also looking problematic and Gentrack is now being forced to take costs out of the business," he said.
Gentrack said contractually recurring revenue was forecast to grow by more than 5 per cent in 2020.
Overall recurring revenue was expected to be flat at about $78m.
Gentrack said the decision by E.ON to suspend its Gentrack implementation is not driven by issues with Gentrack's products or services.
E.ON may restart the project, although it has made no commitment to do so, it said.
The company said its airports business - Veovo - remained a market leader in Australia, New Zealand and the UK for airport billing and operating systems and passenger management systems.
Gentrack, which listed on the NZX in 2014, said it would provide an updated earnings guidance at its annual meeting on February 26.