LONDON, Sept 17 (Reuters) - Britain lowered its national security threat level to severe from critical on Sunday after an investigation into a bomb attack on the London Underground progressed, interior minister Amber Rudd said.

"Following the attack on Parsons Green last Friday, the police have made good progress with what is an ongoing operation," she said.

"The Joint Terrorism Analysis Centre, which reviews the threat level the UK is under, has decided to lower that level from critical to severe."

The level was raised to critical, meaning an attack is expected imminently, on Friday after the bomb injured 30 people in an underground train. Severe, the second highest level, means an attack is highly likely.

Two men have been arrested in connection with the attack. (Reporting by Paul Sandle)