Private equity firm Blackstone Group has made a 1.21 billion pound ($1.68 billion) buyout proposal for St. Modwen Properties, the companies said on Friday, sending shares in the British real estate developer soaring.

Shares in the London-listed firm climbed as much as 20% to 538 pence, just shy of the 542 pence apiece potential offer. They have gained about 30% in the past 12 months.

The non-binding conditional offer, which follows a series of other proposals by Blackstone, was at a premium of about 21% to the stock’s last closing price.

In February, Blackstone joined forces with former Gatwick owner GIP and Bill Gates’ investment vehicle to buy private jet services firm Signature Aviation for $4.73 billion.

Read more at Reuters