Karunanayake is being questioned over a bank take-over attempt in which he was involved, where a well-known foreign investor had remitted money directly into a bank account without going through the specified procedure.
Police also summoned two other UNP lawmakers, Vajira Abeywardene and Earl Gunasekera.
The police summons on lawmakers drew protests from UNP leader Ranil Wickremesinghe who complained to the parliament's speaker that it was a violation of their parliamentary privileges.
Both the government and opposition have been accused of engineering defections ahead of the crucial budget vote due on Monday.
The government, which has only a slim majority in parliament, would fall if it fails to muster enough votes to get its budget passed.
The latest horse trading emerged because a key