Brookfield Infrastructure Partners L.P. is ready to take a run at Inter Pipeline Ltd., saying it’s planning to launch an unsolicited takeover offer worth $5.7 billion for the midstream player.

Under the terms of the proposal announced Wednesday, Brookfield would pay a combination of cash and stock for the stake in Inter Pipeline it does not already own, as the infrastructure company currently holds a 19.65 per cent stake in the pipeline company.

Excluding Brookfield’s stake, the offer values the equity of Inter Pipeline at about $7.1 billion, or $16.50 per share, a 23 per cent premium to Inter Pipeline’s closing price Wednesday.

Inter Pipeline’s TSX-listed shares spiked on Thursday, rising as much as 36 per cent at one point and trading at a  29-per-cent gain of $17.30 as of 12:36 p.m. ET.

The takeover offer potentially pits Inter Pipeline against its largest individual shareholder, with Brookfield disclosing it has made several proposals to buy out the company to no avail before publicly disclosing the bid. According to a release, Brookfield first approached the company about a strategic transaction in September 2020, and ultimately submitted privatization proposals to the board in November and December that it said were rebuffed.

Brookfield said those proposals represented between a 40 and 50 per cent premium to Inter Pipeline’s share price at the time.

In a statement released Thursday, Inter Pipeline acknowledged it had received those conditional proposals, and would review a formal offer if or when one is made.

According to Brookfield, differing views of Inter Pipeline’s intrinsic value stood in the way of a transaction being consummated out of the public eye, with Brookfield claiming that in spite of communications that were “positive in spirit,” the two sides could not agree on the economic prospects of Inter Pipeline in a lower-carbon environment.

Brookfield said it would be open to increasing the offer price, but only on the conditions it be allowed to conduct further due diligence and that the terms of its talks would not preclude it from taking an offer directly to shareholders.

The offer from Brookfield comes about 18 months after Inter Pipeline reportedly turned down a $30 per share takeover proposal from Hong Kong billionaire Li Ka-Shing’s CK Infrastructure, which would have represented a 30 per cent premium for the company at the time.

Since that offer, Inter Pipeline’s share price has posted significant declines, in part due to overall energy market weakness and also amid cost overruns at the company’s $4-billion Heartland petrochemical plant development in Alberta. Heartland is expected to be complete and fully operational in about two years.