Shopping centre group Hammerson has branded a takeover approach from European rival Klepierre "wholly inadequate" and stood by plans to seal a £3.4 billion tie-up with Intu.
France's Klepierre made a proposed 615p-per-share bid on March 8, but saw the move snubbed by the Birmingham Bullring owner in less than 24 hours.
Hammerson shares soared 25% on the London Stock Exchange following news of the £4.88 billion cash-and-share approach.
The move represents a 41% premium of its Friday share price of 437.1p per share.
Klepierre, a French property giant with shopping centres in 57 cities across the globe, said there was no guarantee that a firm offer would be made.
David Tyler, chairman of Hammerson, said the Klepierre proposal was "entirely opportunistic".
He said: "It is a calculated attempt to exploit the disconnect between our recent share price performance and the inherent value of our unique and irreplaceable portfolio which is delivering record results.
"Klepierre is asking our shareholders to accept a price for their Hammerson shares which is not only at a significant discount to their book value but includes a large element of paper in a company which in our view has a lower quality portfolio and lower growth prospects.
"The Hammerson board sees absolutely no merit in Klepierre's proposal and has unanimously rejected it. The board strongly advises shareholders to take no action."
Hammerson agreed an all-share takeover of rival Intu in December last year, which will create Britain's biggest property company.
The acquisition will see Hammerson shareholders owning 55% of the combined firm and Intu investors the remainder.
Intu operates the Trafford Centre in Manchester, while Hammerson owns Bicester Village and Brent Cross shopping centre.
It comes at a tough time for Hammerson, which has been relegated to the FTSE 250 Index after seeing its share price slide in response to market concerns over the woes on the high street.
The bitter trading conditions facing retailers have intensified in recent months, with surging inflation and paltry wage growth prompting consumers to tighten their belts.
Hammerson announced last month a 6.9% rise in net rental income for the year to December 31 to £370.4 million, up from £346.5 million a year earlier.