LONDON — Britain’s Army chief warned Monday that the country’s ability to withstand attack and respond to threats is being eroded by a lack of investment in the military, increasing pressure on the government to boost defense spending.

Gen. Nick Carter said Britain has been left exposed to adversaries such as Russia, which already boast capabilities Britain would struggle to match.

“The threats we face are not thousands of miles away but are now on Europe’s doorstep,” Carter told the Royal United Services Institute. “We have seen how cyberwarfare can be both waged on the battlefield and to disrupt normal people’s lives.”

Carter joins the head of the Royal Air Force, Air Chief Marshal Stuart Peach, in warning that Russia is an increasing threat. Prime Minister Theresa May said last year that Russia had “mounted a sustained campaign of cyber espionage and disruption” against other countries.

Carter’s comments seem intended to pressure Treasury chief Philip Hammond to refrain from further cuts to defense spending, which has been hit hard by government-ordered austerity following the 2008 financial crisis.

Some reports have suggested the government is considering combining elite units of paratroopers and the Royal Marines as part of plan to reduce the number of military personnel by 14,000. That would represent a 10 percent reduction from the current staffing level of 137,000.

Meanwhile, some lawmakers have called for the government to increase defense spending to 3 percent of gross domestic product, from the current 2 percent.

Carter said that without action now, Britain will be constrained in its ability to respond to hostile powers.

“The time to address these threats is now,” he said. “We cannot afford to sit back.”

He called for Britain to greatly increase its ability to project power over land routes to Eastern Europe and said Britain needs contingency plans to deal with a number of potential Russian threats that can be put into place quickly in a crisis.

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