The level had been raised on Friday to the highest - critical - following the bucket bomb blast at Parsons Green Tube station that left 30 passengers injured.
"This government is committed to tackling extremism in all its forms - as the Prime Minister said after the London Bridge attack earlier this year, enough is enough", Mrs Rudd wrote in a press release posted on the Home Office's website.
Britain eased its terrorist threat level Sunday from "critical" to "severe" after police arrested a second suspect in the bombing of a subway train in London.
The 1suspect was arrested in the port area of the southern town of Dover, where ferries depart for Europe under section 41 of the Terrorism Act - which means the arrest was made without a warrant.
Police said they are searching a residential address in Stanwell, Surrey, in conjunction with this arrest.
Islamic State jihadists claimed responsibility for the attack, but Home Secretary Rudd discounted it. She added that it was too soon to know whether the culprits behind the attack were known to the authorities.
Images of the bomb posted on social media appear to show a bucket on fire that had been placed inside a plastic bag close to a rail auto door.
"Military personnel will replace police officers on guard duties at certain protected sites which are not accessible to the public".
He confirmed more police - some armed - would remain in place on the streets and transport networks at the beginning of the week.
London Mayor Sadiq Khan said the British capital "will never be intimidated or defeated by terrorism".