9 January 2015
Last updated at 15:48
Tackling terrorism is a “national priority” and UK security services will get all the resources they need, Chancellor George Osborne has said.
He told the BBC an extra £100m had already been allocated to monitoring Britons going to Syria and Iraq.
It comes as the head of MI5, speaking after the Charlie Hebdo attacks, said three recent UK plots had been stopped.
Meanwhile, the BBC understands two suspects for the attack have been on a UK terror watch list “for some time”.
A source told the BBC that brothers Cherif and Said Kouachi – key suspects for Wednesday’s deadly attack on the satirical magazine – were on the Home Office Warnings Index, meaning they would usually be barred from travelling to Britain.
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My sharpest concern as director general of MI5 is the growing gap between the increasingly challenging threat and the decreasing availability of capabilities to address it”
Director general of MI5
It comes as Mr Osborne told BBC Breakfast the UK was facing the threat of a “more complex plot”.
He said: “So we have got to be vigilant, we have got to have the resources there.
“My commitment is very clear. This is the national priority. We will put the resources in, whatever the security services need they will get, because they do a heroic job on our behalf.”
The financial support for monitoring the “self-starting terrorists” who travel to conflicts overseas after getting “ideas off the internet” was given within the last few weeks, he said.
The chancellor said a united front had to be shown against those who sought to destroy our way of life and that the UK was providing “all possible assistance” to authorities in France following this week’s attack.
Security has been increased on the France/UK border following the Paris attack, in what Home Secretary Theresa May described as a precautionary measure.
Prince Harry has signed a book of condolence at the French embassy in London. The prince was greeted at the embassy in Knightsbridge, central London, by French ambassador Sylvie Bermann, and wrote the message “With warmest best wishes”.
After the prince left, Ms Bermann said: “For us, it’s very important to have the support of the Royal Family. We were very moved by the support of Her Majesty the Queen and then the prince. It shows the solidarity [with us] of the British people and British government and Royal Family.”
The Queen has sent a message to France’s President Francois Hollande in which she expressed her “sincere condolences”.
The book has already been signed by dignitaries including the home secretary and the London Mayor Boris Johnson as well as ambassadors from across the world.
Members of the public can leave a message in a book of condolence at the French Institute in London until 16 January.
The embassy is running an online book of condolence on its website which people can add to by email.
The embassy said: “The French Embassy thanks everyone who has joined the people of France in mourning; your show of solidarity and expressions of condolence have been deeply moving.”
Mrs May will attend a meeting of interior ministers from Europe and the US in Paris on Sunday to discuss the international response following the attack.
Prince Harry expressed his “warmest best wishes” to the French people
London Mayor Boris Johnson has also left a message of solidarity
Mr Osborne’s comments came after the MI5 director general warned that security services could not be expected to stop every plot.
Andrew Parker said the number of Britons who had travelled to Syria was now about 600 – and that a “significant proportion” of them had joined Islamic State militants.
The government had previously estimated that 500 had travelled to fight in Syria.
In the speech at MI5’s headquarters, Mr Parker warned the UK was facing “more complex and ambitious plots” by extremists.
The shootings in Paris were “a terrible reminder of the intentions of those who wish us harm,” he said.
It comes after a number of anti-terrorism operations in the UK in recent months, including three foiled plots in the last three months.
“Deaths would certainly have resulted otherwise,” Mr Parker added.
“But we cannot be complacent. Although we and our partners try our utmost we know that we cannot hope to stop everything.”
Mr Parker said the UK faces a “very serious” and “complex” threat of terrorism
He stressed the UK was not facing an “unmanageable crisis” however, saying “different styles and shapes of terrorism” had been “faced down” for more than 40 years.
Mr Parker, who was named director general of MI5 in March 2013, said the security services knew a group of al-Qaeda extremists in Syria planned “mass casualty attacks against the West”.
But he said the number of “crude but potentially deadly plots” MI5 was facing had also increased.
Gordon Corera, BBC security correspondent
The threat is growing, MI5 is stretched, some of its capabilities are at risk.
All of that means something is likely to happen. That was the bleak message from Andrew Parker.
It was notable that he said the threat comes not just from the self-starters, inspired by the group calling itself Islamic State, but also from groups linked to al-Qaeda still planning mass casualty attacks, including one cell in Syria.
This speech may have been planned before Paris, but events there will add to its impact on a public being told that something similar may well happen here.
Mr Parker warned that changes in technology were making it increasingly harder for security services to intercept communications between extremist groups.
“Wherever we lose visibility of what they are saying to each other, so our ability to understand and mitigate the threat that they pose is reduced,” he said.
Mr Parker’s warning follows recent attacks in Belgium, Canada, Australia, as well as Wednesday’s attack in Paris
Conservative Sir Malcolm Rifkind, chairman of the Commons Intelligence and Security Committee, said the Paris attacks had given “added weight” to the case for intelligence services to get stronger powers to intercept communications.
He told BBC Radio 4’s Today programme it was becoming “increasingly difficult” to access vital evidence.
Sir Peter Fahy, vice-president of the Association of Chief Police Officers, said the threat was two-fold.
Sir Peter, also chief constable of Greater Manchester Police, told BBC Breakfast “In the past, it was just about big plots coming in from the outside.
“Now we’re just as concerned about individuals in this country being rapidly brainwashed on the internet and suddenly deciding to perform some chaotic attack. So we’ve got to be vigilant to both.”