This bid comes from manager Andre Villas-Boas as he made a decisive move to improve his squad before the transfer window closes.
The offer is expected to secure the 23-year-old from Valencia who is likely to accept the bid ahead of interest from Tottenham Hotspur.
Arsenal dropped their interest in Mata last week, having allowed a £22 million buyout clause in his contract to expire, and it now appears that was probably in response to Chelsea’s intervention and the knowledge they would trump any offer.
Villas-Boas is keen to add pace, creativity and width to the team and has agreed that Mata would be an important addition to his squad.
His arrival would leave a huge question mark over the futures of Florent Malouda and Yossi Benayoun, especially as Chelsea also have to create some room in their 25-man squad for new signings who are experienced foreign players such as Mata.
Andre Villas-Boas is also eyeing bids for Porto's Alvaro Pereira and Joao Moutinho, who would cost a combined £40 million.
And the Blues remain interested in Tottenham's Luka Modric, with the club preparing a third bid at £30 million for a player White Hart Lane chairman Daniel Levy insists will not be sold.