Police did not provide further details about the alleged plot. Kerry is due to appear at London's Westminster Magistrates' Court on Wednesday.
Senior national coordinator for Counter Terrorism Policing Vicki Evans said in a statement: "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."
Kerry was charged in July with murdering Widdecombe, 78, a prominent Reform UK figure and former minister. She was found dead at her home in rural south-west England on July 9 after being attacked the day before. Prosecutor Kashif Malik told a court that Kerry allegedly used a hammer to hit Widdecombe on the top of her head 21 times while she was eating lunch, after driving about 500 kilometres from his home in Rotherham. Her gardener found her body face-down on the kitchen floor.
The development comes as Farage faces a parliamentary investigation into whether he should have declared a £5 million ($9.5 million) gift from a billionaire cryptocurrency investor. Farage has said the money was necessary to provide him with security.
A Reform UK source told Reuters: "This is exactly why Nigel needed £5m to protect his security for life." British Home Secretary Shabana Mahmood described the charges as extremely concerning. "My profound sympathies are with Nigel Farage and his family," she said on X. "Let no-one make any mistake: an attack on anyone in politics is an attack on us all. This case raises serious questions about the security of those in public life."