The controversial German wind farm operator Prokon has warned 75,000 retail investors of imminent insolvency, perhaps by the end of the month.

Prokon, based in Itzehoe, Germany, warned in a letter to investors published on the Internet that if they don't waive repayment of the money, it will be bankrupt by the end of January, making it one of the largest bankruptcies ever in the German "grey capital" market, Suddeutsche Zeitung reported.

Consumer advocates have long warned against the company, which spends heavily on online advertising and direct mail solicitations promising incentives of 8 percent returns on its over-the-counter shares, the German newspaper said.

In the letter, the wind farm operator's board appealed to its investors for further financial help.

"If our investors do not succeed, together with you, to stabilize the liquidity situation very quickly, by the end of January, we will probably be forced by law to initiate an insolvency plan for threatened bankruptcy," it said.

The shares are essentially unsecured loans, without the participation rights of regular shareholders -- thus the investors are threatened with a loss of their capital.

Over the years Prokon says it has collected nearly $2 billion through such advertising. It has built wind farms and sold the electricity into the German grid, and also invests in biodiesel and biomass.

For several years its has faced criticism from investors, consumer advocates, the media and prosecutors. They suspected the company could not keep its promise high returns and that investors' money would possibly be lost in the wind farm projects.

Critics have asked where the profits that have been paid out to investors have been coming from -- now those fears could be borne out.

In the letter, the company used dramatic language in an appeal to investors asking them to accept delays in paying out the millions of dollars owed to them under a profit participation plan, the newspaper said.

"An insolvency plan can only be prevented if we receive the agreement of at least 95 percent of the profit participation capital that you will not (demand) your capital at least until Oct. 31, 2014, and a payment within 12 months, which may be paid out in installments," it said.

At the end of the letter, company president Carsten Rodbertus escalated the request into an open call for help.

"Don't let there be an insolvency plan!" he wrote, urging investors to not let "locusts" and negative media reports destroy "a flagship" green energy company.

The Higher Regional Court of Schleswig-Holstein in September upheld an unfair advertising complaint against Prokon made to the government consumers affairs office in Hamburg, Deutsche Welle reported.

It found a company prospectus contained misleading statements about the supposed safety of the advertised participation rights after claiming the investments were "as safe as a savings account."

In December Prokon called on its investors to forgo their returns for the second half of 2013 to ease what it called a temporary liquidity crunch.

Officials of the company could not be reached for comment on Saturday, the German broadcaster said.