A counter-terrorism publicity campaign reminding the public of the threat to the UK has been launched by police.
The two-week campaign features a 40-second radio advert to be broadcast in England and Wales with the message: "If you suspect it, report it."
The Metropolitan Police said everyone had a role to play in fighting the "real and serious" terrorism threat.
A confidential anti-terrorist hotline has been set up for the public to report any suspicious incidents.
A Met Police spokesman said that public vigilance and awareness were "crucial in helping to create a hostile environment for terrorists".
He added: "The advert recognises that some people may be reluctant to report suspicious activity or behaviour, such as a person taking an unusual interest in security arrangements, because 'Chances are, it's probably nothing'.
"But it goes on to encourage people to think 'But what if it isn't?'"
Assistant Commissioner John Yates, head of the Metropolitan Police's specialist operations, said: "Recent events at East Midlands Airport and continued speculation about 'Mumbai-style' attacks serve as a stark reminder of the real and enduring threat we continue to face from terrorist groups.
"These groups are constantly looking at new and inventive ways to launch attacks on the UK and our citizens.
"We all know that the threats against us continue to diversify - from Al Qaeda-led and inspired groups, through Irish-related terrorism to right-wing extremists.
"The police and security services need the help of the public to protect the country from the threat of terrorism. We cannot do it alone. We all have a responsibility to remain vigilant and aware and to report any suspicious activity. We must not become complacent."
The anti-terrorist hotline can be reached on 0800 789 321.