By Wiktor Szary

WARSAW (Reuters) - Polish prosecutors said on Tuesday they would re-open the coffins of all victims of a 2010 presidential jet crash to examine the remains, a move likely to deepen political divisions surrounding the investigation.

The crash near Smolensk in western Russia killed 96 people including Poland's president Lech Kaczynski and his wife, as well as the central bank chief, top army brass and several lawmakers.

An inquiry by the previous centrist government returned a verdict of pilot error, but the ruling Law and Justice (PiS) party led by Kaczynski's twin brother Jaroslaw, says the crash may have been caused by an explosion on board.

Jaroslaw Kaczynski has also repeatedly accused then prime minister Donald Tusk, now head of the European Council, of being indirectly responsible for the crash through negligence.

The prosecutors' decision to exhume all uncremated victims' remains comes months after the government merged the posts of prosecutor-general and justice minister, giving itself more direct control over the investigation.

It also follows the ruling conservatives' decision to relaunch a government investigation into the case. At a ceremony marking the re-launch, Defence Minister Antoni Macierewicz said the plane "disintegrated" meters above the ground.

The move is likely to strain relations with Russia already fragile over the Ukraine crisis. Though PiS never accused Russia of orchestrating the president's death, it has said the Kremlin benefited from the crash.

PiS officials have also accused Moscow of prolonging its investigation, and withholding evidence, such as the black boxes and the plane's wreckage. Russia says these cannot be returned until its criminal investigation is concluded.

The Polish state prosecution said it informed the victims' families that exhumations were necessary, as comprehensive post-mortem examinations were key to reconstructing the chain of events and establishing the cause of the crash, "despite the several years which have passed."

Six of the previously exhumed nine bodies had been wrongly identified, the prosecution said in a statement, adding the re-opening of coffins, which had been sealed in Russia, was necessary to confirm all identities.

The crash took place near Smolensk, western Russia, close to the place where Stalinist secret police forces shot some of the 22,000 Polish officers and intellectuals they executed in 1940. For decades, Moscow blamed Nazi Germany for the mass executions.

The massacre is an enduring symbol for Poland of its suffering at Soviet hands, and president Lech Kaczynski had been flying in to commemorate it.

(Reporting by Wiktor Szary; editing by Ralph Boulton)