It is understood the 31-year-old striker has given Boro boss Gareth Southgate the answer he was dreading and decided not to extend his stay at the Riverside Stadium.
The Magpies have been courting the Australia international for a year and now appear to have made a decisive breakthrough amid interest from the Hammers and Pompey among other suitors.
Viduka's arrival would represent Sam Allardyce's first major signing since taking over as manager at St James' Park.
Meanwhile, Australia coach Graham Arnold is confident Viduka will feature for the Socceroos at the Asian Cup despite the speculation over his future.
Arnold has admitted to "some fears" that Viduka, who is currently on a three-week break in the USA, could pull out of the tournament, which begins on July 8 against Oman in Bangkok.
Viduka missed the midweek Asian Cup warm-up defeat by Uruguay in Sydney but Arnold remains confident he will include the Melbourne-born striker in his 23-man squad for Australia's maiden Asian Cup assault.
"I guess these sort of things cross your mind from time to time," Arnold told The Australian.
"There is always something cropping up with the national team in terms of player availability.
"Mark has a lot on his plate at the moment, so he has every right to take his time to come up with the right decision.
"I spoke to him before the match against China and during my last visit to England, and he was very keen to play. Mark is a man of his word.
"He admitted, however, that he was very tired from a long season and needed a good break, which is why he did not come to Sydney for the Uruguay game."
Celtic-bound Scott McDonald is ready to step into the squad should Viduka withdraw.