LONDON (Reuters) - Britain's security agencies will receive a significant boost to staff numbers as part of a multi-pronged effort to stave off threats from terrorist plotters, finance minister George Osborne said on Monday.

"I can confirm that over the next five years we will substantially increase the number of people across all three secret intelligence agencies who investigate, analyse and help disrupt terrorists plots," he said in a speech.

Osborne added that Prime Minister David Cameron would provide further details at the end of the government's defense spending review on Nov. 23.

The Strategic Defence and Security Review is expected to set out Britain's military capability priorities for the next five years.

The government recently put forward new legislation that would give intelligence agencies sweeping new powers including the right to find out which websites people visit.

"As the nature of war, espionage and terrorism changes, we must change with it. The internet, central to modern life, provides new ways for our enemies to plan and act against us," Osborne said.

"The threat from terrorists, from extreme ideologies, needs to be challenged head on", he said, adding that the "probable fate" of a Russian plane that crashed in Egypt was a painful reminder of that need.

Britain has said that it was "more likely than not" that a bomb brought down the Russian plane shortly after it took off from Sharm al-Sheikh, killing all 224 passengers and crew. Islamic State militants fighting Egyptian security forces in Sinai have claimed responsibility.