Buyout king Stephen Schwarzman’s Blackstone Group has advanced into the final stages of a $4.25 billion auction for half of a chemical company whose biggest shareholder is Bill Ackman, sources told The Post.

Platform Speciality Products is looking to sell the agricultural chemical side of its business, with Blackstone leading a consortium of bidders that’s currently in the lead, sources told the Post.

Platform Specialty aims to sign a deal in the next two weeks to sell the division, whose manufacture of products to protect berries, grapes and beehives represents about half the overall business, a source said.

Ackman’s hedge fund Pershing Square Capital is the leading shareholder, with a 14.5 percent stake.

However, Indian agricultural company UPL Ltd also is considering making a rival offer for the Platform Specialty division, teamed with private equity firm New Mountain Capital, a source said, confirming media reports earlier in the day.

If the sale is completed, Platform Specialty plans to buy back about $1 billion of stock and retire most of its debt, a source following the process said.

The expectation is the remaining half of Platform Specialty’s business, specializing in chemicals that help make circuit boards and cars, would then trade at a much higher multiple, boosting the shares to near the $20 level, the source said.

In October 2014, Pershing bought $241 million worth of Platform Specialty shares for $25.59 each, filings show. However, a Pershing Square rep said the fund’s average cost for its stake is $13.63.

Platform Specialty’s shares were up 5 percent in mid-day trading to $13.56.

On June 21, immediately before The Post broke the news of Blackstone’s interest in the Platform division, Platform shares closed at $12.26.

Meanwhile, Ackman announced Friday he was raising a new Pershing Square VI fund, and is soon expected to announce a new major position, sources said.

A Platform spokesman declined comment.