Neil Entwistle, 27, was arrested in the UK by detectives but it is not clear whether he was taken from his parents home in Worskop, Nottinghamshire, where he has been staying since the bodies of Rachel Entwistle, also 27, and nine-month old Lillian were discovered last month.
His arrest was confirmed by the Middlesex District Attorney's office in the US which is to hold a press conference later today.
A spokeswoman for Nottinghamshire Police refused to comment on Entwistle's arrest saying it is down to authorities in the US.
Entwistle is due to appear before Bow Street Magistrates Court for an extradition hearing today or tomorrow, Scotland Yard said.
The Home Office said the US has issued a provisional request for Entwistle's extradition.
Such requests are issued in "circumstances of urgency". They must then be certified within 60 days, which includes gaining the approval of the British Home Secretary.
"Entwistle was arrested this morning on a provisional warrant relating to an extradition request from the US," a Home Office spokeswoman said.
She added that she understood he had been arrested by the Metropolitan Police at Royal Oak tube station in West London, shortly before noon.
It is thought that Entwistle fled the US before the bodies of his wife and daughter were found at their rented home in the quiet suburb of Hopkinton, Massachusetts on January 22.
He was tracked to Worksop by American detectives a week after the murders, but until today was labelled only as a "person of interest" by authorities.
Rachel and Lillian were found by detectives in an unmade bed. Forensic tests suggest the pair, who had been shot, were killed one or two days before officers found them during a well-being check at the property.
Entwistle's car was later found at Logan International airport in Boston where he is understood to have boarded a flight to London on January 20.
He has since been staying with his parents, Clifford and Yvonne, at their semi-detached home in Kilton, Worksop.
No members of the Entwistle family attended a funeral service held for Rachel and Lillian in Plymouth, Massachusetts, last Wednesday.
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