FRANKFURT, Sept 20 (Reuters) - Germany's government is nearing an agreement about nationalisation of utility Uniper , the most high profile corporate casualty of Europe's energy crunch, sources said on Tuesday while official confirmation from Berlin, Uniper and parent Fortum was not available.

Uniper alleges that its long-term Russian partner Gazprom has brought it to the brink of insolvency by withholding gas, prompting 19 billion euros ($18.97 billion) of support pledges from the government, which still may not be enough.

Uniper has described itself as a "pawn" in the crisis triggered by Russia's invasion of Ukraine.

Russia three weeks ago closed the important Nord Stream 1 export pipeline into Germany, citing Western sanctions as hindering operations but European politicians call the argument a pretext for weaponising gas supply.

Here is how Uniper ended up in such dire straits.

SEPTEMBER

Uniper hints at a majority participation by the German government, as the government bailout to date looks not to be enough to cover the cost of replacing falling gas receipts from Russia, and shares fall to all-time lows.

Uniper relied on long-term supply contracts with Gazprom along with sector peers SEFE - formerly known as Gazprom Germania - and EnBW subsidiary VNG. All three may benefit from proceeds of a planned gas levy from Oct. 1.

Uniper is weighing legal proceedings against Gazprom in a Swedish arbitration court.

AUGUST

Germany sets the gas levy that would allow Uniper and its rivals to pass on 90% of spiralling gas prices to customers.

JULY

The German government agrees to a 15 billion euro Uniper rescue deal and to more than quadruple a credit line with state-lender KfW.

Germany commits itself to raising money via the gas levy, to fund bailouts and uphold gas distribution.

JUNE

Uniper withdraws 2022 outlook and calls for bailout.

MAY

Uniper implements Moscow's demand to pay for gas in roubles which the European Commission said could breach sanctions.

APRIL

Uniper hit by 3 billion euros first quarter net loss, posts impairments related to its Russian generation unit.

MARCH

Uniper tries to appease investor concerns by signalling exit from Russia and sale of its 83.7% Unipro stake.

FEBRUARY

Uniper says it will propose a 95% cut in its 2021 dividend.

The Nord Stream 2 pipeline link in which it was a co-financier, is abandoned amid Russia's invasion of Ukraine, leaving a 1 billion euro write-down. NS 2 would have doubled the NS 1's capacity of 55 billion cubic metres a year.

Russia on Feb. 24 invades Ukraine, calling it a "special military operation" to disarm its neighbour.

JANUARY

Uniper secures credit facilities worth 10 billion euros from main shareholder Fortum and German state bank KfW plus 1.8 billion euros in bank loans in a precautionary move to address high prices and volatility.

2021

Global gas markets became tight in a post-COVID recovery and Russia started sending less westwards, pointing to the imminent start of the Nord Stream 2 pipeline.

Uniper, which was feeling stress from a shift to decarbonisation that forced the shutdown of coal plants, maintained throughout that Russia was a reliable shipper.

LEGACY

Uniper represents the legacy business of Ruhrgas which E.ON took over in 2003 and fully absorbed in 2013.

The purchase which cemented joint German-Russian trade and hydrocarbon exploration links was opposed by the cartel office and by some as pandering to pro-Moscow business interests.

E.ON spun off a majority of Uniper in 2016 and later agreed to sell its remaining stake to Fortum, which now holds 78%. ($1 = 1.0017 euros) (Reporting by Vera Eckert, editing by Miranda Murray)