WARSAW (Reuters) - The leader of Poland's ruling Law and Justice (PiS) party, Jaroslaw Kaczynski, will return to the cabinet as deputy prime minister, the government spokesman said on Wednesday, ahead of a closely fought election expected in October or November.

The return of Kaczynski, widely viewed as Poland's de-facto ruler, comes amid a stuttering campaign by the conservative ruling party for a third term that would likely see it continue policies that have set it at odds with the European Union.

"The head of our political camp, PiS leader Jaroslaw Kaczynski, will become the only deputy prime minister. The appointment will take place today," spokesman Piotr Muller told state news agency PAP.

"The remaining deputy prime ministers have resigned from the functions of deputy prime ministers and will remain as ministers," he added.

President Andrzej Duda is expected to formally approve the changes later on Wednesday.

Kaczynski previously served as deputy prime minister from October 2020 till June 2022, and also headed Poland's security committee.

Although PiS is ahead in most opinion polls, a question mark remains over whether it can secure a parliamentary majority.

Opposition parties received a boost this month from a large anti-government march in Warsaw.

PiS faces rising anger over Poland's strict abortion laws and was strongly criticised for a decision to use images of Nazi Germany's Auschwitz death camp in a campaign video.

Divisons in the ruling coalition have also re-emerged over Poland's relations with Brussels, after the EU withheld funding over rule of law concerns. Observers say Kaczynski's return may help to smooth those rifts.

Critics say PiS, since sweeping to power in 2015, has undermined the independence of Poland's courts, turned state media into an outlet for propaganda and stirred up discrimination against ethnic and sexual minorities.

PiS denies subverting any democratic rules and says its policies are aimed at preserving Polish traditions and at making the economy work in a fairer way for all Poles.

Its mix of conservative social values and generous social welfare programmes has proved popular with many voters, propelling it to victory in two consecutive parliamentary elections.

(Reporting by Pawel Florkiewicz and Karol Badohal and Alan Charlish; Editing by Gareth Jones)