According to a report in sports daily A Bola — who said an anonymous complaint had been made to police on Thursday — officials from the two clubs had met in a Lisbon hotel to negotiate the result of the match played to a completion on February 21.
In the days following the game they received a payment of some 730,000 euros ($900,000), A Bola said.
The game had been abandoned at half-time in January due to fears over an unsafe stadium at Estoril, near Lisbon, with Porto trailing 1-0.
The second half was played on February 21, with Porto coming back to win 3-1.
“It is totally false that there was any negotiation over the second half of the game against Estoril,” said Porto in a statement.
Estoril labelled the claims as “defamatory”.
Francisco Marques, Porto’s communications director, then took to social media to explain that there had been transfers of money towards several clubs including Estoril.
However, he said these had been made to honour debts from transfers of two players whose rights belonged in part to Estoril.
“The transfer of 784,000 euros (allowed) to pay a bill dating to October 26, 2017 for the transfer of Carlos Eduardo to (Saudi Arabian side) Al Hilal and for Lica” to Porto, he explained.
Contacted by AFP, a spokesman for Portugal’s public prosecutor confirmed that a complaint had been received “in connection with the second half of the Estoril Praia-FC Porto match”, and that it was being investigated.
Porto’s bitter foes Benfica were forced to deny similar allegations in December, saying they had been targeted in a “desperate campaign” by their rivals.
Porto are currently five points clear of Benfica at the top of the table.