LONDON — Britain can no longer ignore that its influence on the world is shrinking amid the rise of developing nations, a panel of senior lawmakers said Thursday in a sharply critical report.

The group also said Britain should prepare for looser ties with the United States as the U.S. shifts its focus from western Europe.

Parliament’s Joint Committee on National Security Strategy, which includes an ex-head of the domestic MI5 spy agency, Eliza Manningham-Buller, rebuked Prime Minister David Cameron’s government over its failure to accept that the nation’s power is in decline as austerity measures trim the nation’s military and dent diplomatic ranks.

Britain’s national security strategy, published in October 2010, insisted that the U.K. would maintain its position as a major world power, despite budget constraints and the shift of economic might from the West to the developing world.

“This is wholly unrealistic in the medium to long term and the U.K. needs to plan for a changing, and more partnership dependent, role in the world,” the committee said in its report.

Cameron is making spending cuts worth about $162 billion through 2017 aimed at slashing Britain’s budget deficit, including an 8 percent reduction to its annual $58.8 billion defense budget over four years.

The panel of House of Commons and House of Lords members also urged Cameron to “reflect deeply on the long term implications” of the U.S. turning its attention away from Europe.