Britain is facing an ‘almost inevitable’ attack by fanatics who have been ‘militarised’ by Islamic State, according to police and security officials.

In speeches today, Theresa May and senior police will warn that the ‘diverse’ terrorist threat posed by jihadis returning from Syria and Iraq is one of the greatest this country has ever faced.

Potential attacks could range from a ‘lone wolf’ beheading in a crowded shopping centre or street, to a bomb plot using fertiliser stolen from British farms. One Whitehall official told the Mail: ‘It is almost inevitable that something is going to happen in the next few months.’

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Theresa May will warn that the ‘diverse’ terrorist threat posed by jihadis returning from Syria and Iraq is one of the greatest this country has ever faced. Britain’s most senior officer, Sir Bernard Hogan-Howe, said police and MI5 have already foiled five major plots this year

According to Britain’s most senior officer, Sir Bernard Hogan-Howe, police and MI5 have already foiled five major plots this year. Chillingly, there is normally only one major plot disrupted every 12 months.

Addressing a conference in London, Mrs May will unveil draconian new laws to try to protect the public and stem the flow of cash and recruits to Islamic State. They include a ban on the payment of any ransoms to terrorists and making it easier to track extremists on the internet.

Internet companies will be forced to keep data which helps to identify the user of an individual mobile phone or computer. But the law stops short of making them log every website a person visits after the Liberal Democrats said the so-called ‘snoopers’ charter’ was ‘dead and buried’.

The Home Secretary’s new Anti-Terrorism and Security Bill, to be presented to MPs tomorrow, also includes powers to:

Forcibly relocate terror suspects, or put them into internal exile; allow border guards to seize the passports of suspected jihadis;

Ban fanatics returning from Syria from entering the UK for up to two years;

Forbid airlines from landing in the UK if they do not provide advanced passenger lists.

Sir Bernard said the challenges faced by police and MI5 in monitoring jihadis returning from Syria were huge.

He warned: ‘They’re going to be militarised, they will have a complex web of people that they know, and of course they will have learnt tactics that they may want to use here.’

Speaking on the BBC’s Andrew Marr Show, Sir Bernard said there was a ‘growing concern’ about the risk of a ‘lone wolf’ attack on British streets.

Soldier Lee Rigby was murdered by two Muslim converts who ambushed him as he walked back to his barracks in Woolwich, South East London, last year. ‘It doesn’t take an awful lot of organising, doesn’t take too many to conspire together, there’s no great complexity to it,’ Sir Bernard said. ‘So that means we have got a very short time to interdict, to actually intervene and make sure that these people don’t get away with it.’

Today assistant commissioner Mark Rowley, the Met’s most senior counter-terrorism officer, will say the ‘danger posed by violent extremists has evolved’.

Speaking alongside Mrs May at the Royal United Services Institute, he will say: ‘They are no longer a problem solely stemming from countries like Iraq and Afghanistan, far away in the minds of the public.

‘Now, they are home-grown, in our communities; radicalised by images and messages they read on social media and prepared to kill for their cause.’ He will also reveal how police have offered specific advice to farmers to ensure that fertilisers are stored securely so they cannot be stolen by fanatics to make bombs.

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A member loyal to the Islamic State (ISIS) waves the terror group's flag in Raqqa, Syria

Mrs May, who has been fighting a long-running battle with the Lib Dems for greater counter-terrorism powers, is equally bleak in her assessment.

She said: ‘There was a time when people were looking at perhaps just Al Qaeda-related threats that were a long time in the planning.

‘Now the threat is more diverse in terms of the number of groups out there who will be looking to carry out attacks in the West. It’s also more diverse in that not everybody’s affiliated to a particular group – some are self-starting groups and you do have individuals, perhaps the “lone wolves” or the volatile individuals.

‘We have to look across the board at all sorts of threats that are out there.’

Abu Abdullah Al-Habashi (right), who fled the UK for the Middle East, is believed to have been killed there

The plans to make it easier to monitor the public’s use of the internet revives a long-running controversy which has split the Coalition. At the moment, only the so-called Internet Protocol address is recorded when a person uses the web.

The IP addresses are not tagged to individuals and can regularly change, even on a home computer which connects to the internet using broadband. Under new laws, the internet companies must link these IP addresses to the identity of the person using them.

But, at the insistence of Nick Clegg, it stops short of Mrs May’s demands for a full ‘snoopers’ charter’ which would make internet companies log details of every website a person visits, along with their use of social media.

Tomorrow, a long-awaited report by Parliament’s Intelligence and Security Committee will say MI5 was in the dark about clues on social media that could have alerted them to the fanatical intentions of the killers of Fusilier Rigby.