Poland’s conservative opposition won the parliamentary election with more than a third of the vote, according to provisional official results. The Law and Justice party made strong gains in all major cities and won all but two regions, securing 37.6% of the vote, with the ruling Civic Platform second on 24.1%.

Paweł Kukiz, a rockstar turned rightwing politician, scored 8.8% on Sunday, giving him at least 30 seats in the 460-seat parliament.

Two other parties or groupings also reached the threshold for seats in parliament – pro-market Nowoczesne (7.6%) and the leftwing post-communists of PSL (5.1%).

The announcement of the provisional result was delayed for more than four hours by irregularities in the “protocol” sent back from London covering the results from UK polling stations.

The results are unlikely to change, though, apart from by tiny margins. Senate results – using a first past the post system – are expected on Tuesday.

It was not immediately clear whether the result, which will be made final on Tuesday, would allow the Law and Justice party – led by Jarosław Kaczyński, the twin brother of Poland’s late president Lech – to rule alone.

If that result is confirmed, the party has scored the biggest victory for a single party in terms of seats since Poland shed communism in 1989. It would be returning to power after eight years.

The presence of three smaller parties or groupings in parliament might make for some political horse-trading over the next few weeks but will not weaken the decisive swing towards Law and Justice’s brand of social conservatism mixed with left-leaning economics.

The European commission and Germany, which both had strained ties with the last Law and Justice-led government that fell apart in 2007, said they hoped for good relations with the new government. It will not be led by the combative Kaczyński but by Beata Szydło, his party loyalist, who has little in the way of foreign policy credentials.

The party favours a sharp rise in public spending and a larger state role in the economy. It also wants the central bank to launch a cheap lending programme worth 350bn złoty (£59bn) over six years to support growth – an idea that some see as undermining the bank’s independence.

Kaczyński’s party revealed plans to reap new revenues from next year with a tax on banks’ assets, and there were also signs it was confident of sufficient informal support in parliament from other parties to make changes to Poland’s constitution.

“Many party leaders have talked of wanting deeper change in Poland, so, if we want to deliver that, changes to the constitution are vital,” the party’s spokesman on economic affairs, Zbigniew Kuźmiuk, said on Monday.

Rating agency Standard and Poor’s said the outcome of the vote had no immediate impact on Poland’s A-rating with a positive outlook, but it added that policy measures planned by Law and Justice could dampen investor confidence.

Poland’s main stock market index rose 0.2% on Monday, but shares in some banks fell. The złoty weakened slightly as investors had already priced in a Law and Justice party victory in recent weeks.

Reuters contributed to this report