The current threat to the UK from jihadists will remain for "several years", Britain's chief counter-terror officer has warned.

At the start of Counter-Terrorism Awareness Week, Scotland Yard's Assistant Commissioner Mark Rowley told a press conference the step-change in the extremist threat will continue even if the violence in Syria and Iraq subsides.

Later, the Home Secretary will unveil a package of changes designed to tighten up protections in the UK, amid warnings that the threat of an attack here is growing.

Among the measures to be announced by Theresa May are plans to ban insurance companies from footing the cost of terrorist ransoms.

The Counter-Terrorism and Security Bill, being introduced to Parliament this week, will make clear it is an offence for firms to reimburse such payments.

Asked if the current terror climate would continue for at least five years, Mr Rowley said: "We have seen a step- change that will sustain for many years.

"Even if the awfulness that's happening in Syria and Iraq was miraculously to get sorted in the next year or so - and that looks a very optimistic 'if' - there are other countries across parts of Africa and elsewhere in the world which are in parlous states and the potential for this type of terrorism to reach back into Europe to continue in other theatres is equally great.

"Whether it continues in that theatre or whether it moves into other places, I think there's a high prospect of it continuing in this nature for several years."

Yesterday, Scotland Yard Commissioner Sir Bernard Hogan-Howe disclosed that "four or five" plots have been foiled this year.

Up and down the country, officers will be briefing more than 6,000 people at 80 venues including schools, universities, airports, shopping centres, cinemas and farms in a bid to engage the public and businesses in helping to prevent attacks.

The terror threat level in the UK was raised from substantial to severe a few months ago against a backdrop of increasing concerns over hundreds of aspiring British jihadis travelling to Iraq and Syria to learn terrorist ''tradecraft''.

Over the weekend it emerged that police are to be given powers to force internet firms to hand over details which could help identify suspected terrorists and paedophiles.

The Counter-Terrorism Bill will oblige internet service providers (ISPs) to retain information linking Internet Protocol (IP) addresses to individual users.

However, Mrs May has complained that the Liberal Democrats are blocking wider data collection measures - after they declared so-called Snooper's Charter legislation "dead and buried".

Terrorist groups linked to Islamic State (IS), also known as Isil or Isis, in Syria and Iraq have been using kidnappings to raise funds - with the UN estimating the tactic has brought in £28 million over the past year.

But the Home Office says there has been an element of "uncertainty" about whether UK-based insurers were prohibited from paying claims made by companies and families who had met ransom demands.

Mrs May said: "Our position is clear - ransom payments to terrorists are illegal under UK and international law.

"Agreeing to meet the demands of barbaric groups like Isil would only put many more lives at risk. These measures will ensure the UK remains at the forefront of global efforts to put an end to the practice."

Speaking at the start of the awareness-raising campaign, Mr Rowley, national policing lead for counter-terrorism, said police officers and other law enforcement agencies "cannot succeed alone" in defeating the terror threat and urged the public and businesses to help.

Mr Rowley revealed that, through the Government's Prevent anti-radicalisation programme, counter-terror officers had received 77 reports from families this year, some of which have enabled police to catch aspiring terrorists.

He called on the public to be vigilant and report suspicious activity and urged businesses to check security measures are effective.

He said in recent months the police had seen a "substantial uplift" in suspicious activity reports.

Elsewhere in the UK, police officers and theatre groups will be speaking to students about the Prevent strategy, which provides help to stop people from being radicalised.

Meanwhile, sniffer dogs will be hunting large amounts of money at ports, airports and railway stations to prevent cash leaving the UK for terrorist purposes.

Other events will follow, including work with farmers to ensure that fertilisers - which can be adapted into explosives - are stored securely, and charities advising people about safe ways to donate money.

West Midlands Police Assistant Chief Constable Marcus Beale will address an audience of business representatives in Birmingham this morning.

He will say: "Experience from around the world shows us that terrorists will target busy, crowded places to ensure that attacks have a maximum impact.

"Businesses, particularly those in town and city centres, have an invaluable role to play in preventing potential attacks. Staff working in shops and entertainment centres are often the first people to spot signs that something is wrong."

British Transport Police's (BTP) new Deputy Chief Constable, Adrian Hanstock, will address rail industry representatives at the National Railway Museum in York.

All week at railway stations across England, Scotland and Wales, BTP officers will be providing counter-terrorism information to passengers and delivering training sessions to rail staff.

In the North West, a briefing on protecting coastal and maritime environments will take place at Brunswick Dock in Liverpool.

Assistant Chief Constable Ian Wiggett, the North West lead for counter-terrorism, said: "This is not about scaring people, but highlighting that everybody, from law enforcement to businesses, to the general public, has a role to play in keeping the UK safe from terrorism.

"We encourage people to be vigilant to things that are out of place or suspicious and report it to the police."

IS has posted a series of videos online showing the separate murders of US journalists James Foley and Steven Sotloff, US aid worker Peter Kassig and two British aid workers, David Haines and Alan Henning.

Footage claiming to show Mr Henning's murder appeared on the internet just days after the UK joined US-led air strikes against the terrorists in Iraq.

In addition, an IS fighter with a British accent has appeared unmasked in other films encouraging ''brothers'' in Western countries to ''rise up'' and commit acts of terror in their home countries.

America's Federal Bureau of Investigations (FBI) has launched an appeal for the public's help in identifying individuals who have travelled - or are planning to travel - overseas to engage in combat alongside terrorist organisations.