The firm behind Heinz ketchup was turned down for a bumper £115billion takeover of the Angle-Dutch company, which owns a number of consumer brands, including Dove and Domestos.
The deal would would be one of the biggest mergers of a British firm in corporate history.
An announcement by Kraft Heinz suggested it had not been deterred by the rebuff.
In a statement it said: "While Unilever has declined the proposal, we look forward to working to reach agreement on the terms of a transaction.
"There can be no certainty that any further formal proposal will be made to the board of Unilever or that an offer will be made at all or as to the terms of any transaction."
Experts said the firm could have been spurred on by the weaker pound, with the currency around 15 per cent down against the dollar compared to this time last year.
But a deal could mean British job losses and less competition for consumers.
Naeem Aslam, chief market analyst, said: "Theresa May has not triggered the Brexit yet, the effects of this have started to surface.
"Perhaps, predators see a lot of blood and opportunity and falling sterling has produced enough blood on the street for firms around the world to look and cash in on opportunities.
"They are coming out with big bazookas and trying to make deals which we have not seen.
"You can not blame them because it is cheap for them and the time is right.
"So we do not think Kraft Heinz is going to back down anytime soon.
"If the deal does see the daylight, this simply mean more job loss for UK and more pain for consumers as competition will erode."
Unilever's share price surged by 12 per cent following the news of the offer, which represented a premium of 18 per cent of its closing share price on February 16, valuing the company at $143 billion (£115 billion).
Steve Clayton, fund manager at Hargreaves Lansdown, said: "Putting portfolios of brands together can create huge synergies across marketing, manufacturing and distribution, even before you think about cutting the combined HQ back to size.
"Kraft Heinz are attempting a massive push on the fast forward button, for to acquire the sheer scale of brands that Unilever represents through one-off acquisitions could take decades.
"With debt cheap and abundant right now, Kraft have spotted their opportunity."
Kraft took over Cadbury's in 2010 and then merged with Heinz in 2015.