The consumer watchdog plans to oppose the proposed takeover of Woolworths service stations by BP Australia, saying the deal would substantially weaken competition.

The Australian Competition and Consumer Commission on Thursday said fuel prices would probably increase at the Woolworths sites if BP buys them, while other retailers would face less pressure.

Woolworths currently operates 531 sites and has 12 sites in development, while BP supplies fuel to about 1,400 BP-branded service stations throughout Australia, setting fuel prices at roughly 350 of them.

Following a nine-month review of the proposed acquisition, the ACCC chairman, Rod Sims, said Woolworths was a “vigorous and effective” competitor that had an important influence on fuel prices and price cycles in many markets.

“Many consumers seeking out cheaper petrol will head to Woolworths petrol stations,” Sims said. “The bottom line is that we consider motorists will end up paying more, regardless of where they buy fuel, if this acquisition goes ahead.”

Sims said the acquisition would have a “major impact” on drivers and likely affect metropolitan price cycles by making the price jumps quicker, larger and more coordinated.

“Reduced competition will also mean that prices will not fall as far, or as quickly, in the discounting phase of the cycle,” he said.

Woolworths said in a statement it was disappointed with the ACCC’s decision.

“We have worked closely over many months with BP and the ACCC through an extremely rigorous and demanding process,” it said. “BP and Woolworths Group will assess the options over the coming period.”

In a separate decision, the ACCC has also decided to grant applications from Woolworths and BP, subject to conditions, in regards to shopper docket and rewards loyalty program.

BP and Woolworths must limit shopper docket and loyalty scheme discounts to no more than 4c per litre (in total per fuel purchase) and Woolworths is not permitted to fund more than 2c of the discount.

