Joshua Kerry, 28, is charged with one count of preparation of terrorist acts relating to alleged activity between 31 May 2024 and 8 July 2026, Counter Terrorism Policing said.
Reform UK spokeswoman and former Tory minister Ms Widdecombe, 78, died on July 8 after she was hit over the head with a hammer 21 times at her home in Haytor on Dartmoor, Devon, a court previously heard.
Kerry, of Byrley Road, Rotherham, South Yorkshire, is also accused of her murder.
Senior national coordinator for Counter Terrorism Policing (CTP), Deputy Assistant Commissioner Vicki Evans, said: "We have been running a hugely intensive and complex investigation for some time.
"The charge announced today is a result of the tireless work of investigators across the Counter Terrorism Policing network.
"This investigation has been led by a dedicated team of detectives, analysts, and forensic experts, working on multiple lines of enquiry, which has resulted in this very serious charge."
Kerry is due to appear at Westminster Magistrates' Court on Wednesday.
In August, CTP reopened a year-long probe into an alleged firebomb attack at Mr Farage's home in south London.
The Metropolitan Police and CTP London said they investigated the incident as an "attempted burglary", but made no arrests before the case was closed pending any new lines of inquiry.
Following its reopening, CTP London identified a line of inquiry which it said "may have been relevant" but had not been "identified and pursued by them at the time".
The Reform UK leader told the Daily Telegraph in April that a lit incendiary device was pushed through his letterbox early last year in what he described as an "outright arson attempt".
He was away from his home at the time and discovered the damage when he returned, saying the device had "luckily" burned itself out in the porch.
Police investigated but had not identified any suspects, Mr Farage told the newspaper.
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