The push is aimed at ratcheting up pressure on UK-based Centamin before a month-end deadline by which Endeavour must decide whether to pursue the deal or scrap it entirely.
"Our friendly approach, confidential to the board, was refused despite the merit we see in this transaction," Endeavour Chief Executive Officer Sébastien de Montessus told Reuters on Thursday.
"We are now reaching out to Centamin shareholders so that they don't get denied the opportunity of this combination."
Current takeover rules in the UK give West Africa-focussed Endeavour until Dec. 31 to drum up support, pursue a hostile deal, request an extension or abandon the plan.
Centamin said earlier this week the takeover proposal did not offer any benefits for its shareholders.
Endeavour's approach values Centamin at 126.27 pence a share, while the stock has a median target price of 132.50 pence according to Eikon data from Refinitiv.
Some Centamin investors and industry analysts have said Endeavour will have to offer a richer bid to clinch a deal. Endeavour's de Montessus declined comment on whether it would sweeten its offer.
Gold miners have bulked up with a record $30.5 billion in deals this year to replace dwindling reserves and win back disgruntled investors.
Combining with Centamin would make Endeavour more attractive to funds and shareholders who have called for consolidation in the sector, de Montessus said.
Endeavour is eyeing control of Centamin's Sukari mine in Egypt and has said the combined entity would be one of the world's largest and lowest-cost miners.
Centamin Chairman Josef El-Raghy urged shareholders on Wednesday to take no action on the proposal, saying it "sits in stark contrast with Centamin's strategy."
Centamin stock, which has lagged Endeavour's over the last two years, jumped nearly 15% to 128.8 pence on Tuesday after Endeavour's bid. Endeavour fell 4% in that session.
Centamin shares fell 6.6% to close at 119.35 pence in London on Thursday and Endeavour's shares were down 1.6% at C$24.74 in Toronto on Thursday afternoon.
For a graphic on Centamin soars after Endeavour reveals merger proposal:
(Reporting by Jeff Lewis in Toronto and Muvija M in Bengaluru; Editing by Sweta Singh and Matthew Lewis)