LONDON (Reuters) - The British police, backed by specialist armed officers, arrested three men in central England on Tuesday on suspicion of being involved in preparing acts of terrorism.

Counter-terrorism officers detained one man, in his 20s, at a business address in Letchworth, Hertfordshire, shortly before 1pm. Two others, aged in their 20s and 30s, were arrested at houses in nearby Luton shortly afterwards, London Police said in a statement.

The arrests are unusual as British police usually carry out early morning raids in counter-terrorism investigations if the action is not urgent. Armed officers were present for one of the arrests but no shots were fired.

"Today's arrests and searches are part of an ongoing investigation into Islamist-related terrorism," the statement said. "As this is an active investigation, we are not able to release any further information at this stage."

Britain is on its second-highest alert level of "severe", meaning an attack is considered highly likely, mainly due to the danger the authorities say is posed by Islamic State fighters and Britons who have joined them.

Last year there were more than 330 arrests for counter-terrorism offences.