The contract was cancelled earlier this month by the Law and Justice (PiS) government, which has strong conservative and eurosceptic views and has challenged many decisions made by its centrist predecessors.

A spokesman from Airbus Helicopters declined to comment.

PiS has angered fellow NATO member France and unsettled foreign investors by saying the deal would not benefit the Polish economy and accusing the previous cabinet of foul play.

Prosecutor Michal Dziekanski told state news agency PAP on Friday his office will check whether officials had exceeded their authority or failed to perform their duties in negotiating the deal, as well as in scrapping it later. They will also investigate potential bribery, Dziekanski said.

"This will be a complicated, comprehensive investigation, encompassing a very large set of evidence," Dziekanski told public broadcaster TVP Info.

Centrist defence minister Tomasz Siemoniak, whose Civic Platform party lost an election to PiS last year, has called the tender "fair and transparent" and said it was cancelled by his rivals for political reasons.

The row over the tender has added to mounting displeasure with Poland in the EU over issues such as migration and the rule of law at a time of increased Russian assertiveness in eastern Europe.

Under the provisional deal, Airbus was to deliver 50 Caracal multi-role helicopters for a total of 13.5 billion zlotys (£2.81 billion) for Polish armed forces, including special forces, anti-submarine and sea rescue units.

The price per Caracal was to be higher than paid by Kuwait, which in August ordered 30 Caracal helicopters for 1.1 billion euros (£988.62 million).

Some analysts said the higher price for Poland was due to special requirements for equipment, training and local investment.

However, former Polish president Aleksander Kwasniewski told local media there was a "gentlemen's agreement" among EU and NATO members to look more favourably on French defence offers after it agreed not to sell its warships to Russia following Moscow's annexation of Ukraine's Crimea peninsula in 2014.

(Reporting by Marcin Goettig; Editing by Ruth Pitchford)