US Edition The offer is worth 70p for each EnCore share, with the payment available in either cash or Premier shares. This represents a 55pc premium to EnCore's closing price on Tuesday, but it is below the 12-month high of 151½p achieved in February.
A successful takeover would increase Premier's stake in the Catcher discovery to 50pc from 35pc and give the company operator status. Catcher, located 110 miles south-east of Aberdeen, is one of the largest recent discoveries in the North Sea.
"Operating and controlling a key asset will make sure it goes ahead at our pace," Tony Durrant, Premier's chief financial officer, told The Telegraph. The deal also brings some exploration assets into its portfolio.
EnCore shares rose 28¾ to 74p, which is above Premier's offer price as there is a chance that a counter bidder could emerge.
However, EnCore directors have agreed to sell their 7.4pc stake in the group to Premier and recommended the deal to shareholders. BlackRock, EnCore's largest shareholder, has also tendered its 5.8pc holding in the company.
"The acquisition de-risks EnCore's development portfolio of assets... and also gives EnCore shareholders the option of retaining exposure to EnCore's assets within the enlarged portfolio via the share alternative," Alan Booth, EnCore's chief executive, said.
Premier said it will use existing cash balances to finance the deal.
Analysts broadly welcomed the combination for reducing the risk around the Catcher development.
"We view Premier's recommended acquisition of EnCore positively as it reduces the partner risk around Catcher," Phil Corbett, an oil analyst at RBS, said.
Premier shares fell 2.8 to 335.9p.