By Tom Bergin

LONDON, Sept 8 (Reuters) - BP is due to release itsinternal investigation into what caused a rig blast that led tothe United States' worst ever oil spill on Wednesday andinvestors will be looking for clues as to whether BP will beable to fend off accusations of gross negligence.

BP's partner in the blown-out Gulf of Mexico well, AnadarkoPetroleum, has accused it of gross negligence.

If this is proven, BP will be liable for 100 percent of thecosts of cleaning up the spill, rather than 65 percent it wouldotherwise pay, the level equal to its ownership of the well.

If gross negligence is proven, BP could also be liable tofederal fines of over 20 billion dollars, as opposed to under aquarter of that if this is not proven.

BP has said it believes it was not grossly negligent andprevious statements from it and testimony from survivors toofficial probes suggest representatives of all the companiesinvolved will be criticised on Wednesday.

BP has previously criticised rig operator Transocean andHalliburton, the company which cemented the inside of the well.Investigators believe the cement job probably failed, allowinggas into the well, which subsequently came up onto the rig andcaused the blast.

But if the report -- which BP said was compiled by its headof safety, Mark Bly, without influence from senior managementproduces evidence of gross negligence -- it could spookinvestors.However analysts do not expect this.

"We believe (the report) could shift the focus ofculpability back towards Transocean and in particular theintegrity of the Blowout Preventer (BOP) which should have actedas the ultimate fail safe," Keith Morris, oil analyst atEvolution Securities said in a research note.

"If our view is correct then BP's shares could rally thisafternoon as expectations of gross negligence litigation iseroded".

Yet, if BP seeks to shift most of the blame onto Transocean,which was responsible for the BOP, it risks being accused ofmerely seeking to reduce legal liability, rather than unveil thetruth.

President Obama previously criticised blame-shifting by thecompanies, so if BP does dump more blame on others it couldfurther depreciate BP's already low currency in Washington,where it needs to curry favour if it wishes to continueoperating in the Gulf of Mexico.

The report will be closely watched to see how far up the BPmanagement chain it finds fault.

If BP only finds low-level employees to blame, it will beaccused of a whitewash.

After its Texas City blast in 2005 when 15 workers werekilled, BP sacked several hourly workers, blaming them forfailing to follow correct procedures.

The official probe by government regulators later blamedmore structural problems at BP, most notably a focus on costsaving over safety.

So far many U.S. politicians have already concluded that therig blast was due to similar cost-cutting efforts by BP.Accusations of bad faith are likely to be enhanced ifemployees who failed to assist federal regulators are found tohave contributed to the BP report. (Reporting by Tom Bergin; Editing by Hans Peters)