As the U.K. prepares to host heads of state and government and military leaders gather for a North Atlantic Treaty Organization (NATO) summit in London next week, several warnings have emerged over limitations of its own warfighting capability, especially if there was a conflict with Russia.

A report from the country's Royal United Services Institute (RUSI), a prominent defense think tank based in London, said on Wednesday that U.K. ground forces would be "comprehensively outgunned" in a potential conflict with Russia in Eastern Europe.

The paper, written by RUSI Research Fellow Jack Watling, called "for the modernization of its fires capabilities to be a top priority" warning that the U.K. currently 'possesses a critical shortage of artillery." Artillery has historically inflicted between 60-90% of casualties in high-intensity warfare, the report noted.

The RUSI paper published Wednesday was aimed at assessing the U.K.'s "commitment to the (NATO) Alliance's capabilities in a potential high-intensity conflict with Russia, in which the U.K. has promised to deliver a warfighting division."

However, in benchmarking against Russian capabilities, the paper stated that "the U.K.'s ground forces are comprehensively outgunned and outranged, leaving enemy artillery free to prosecute fire missions with impunity."

Although he noted that "direct war with Russia is unlikely," Watling said that for the U.K. to regain a competitive edge "it must either be prepared to invest in highly effective but expensive stockpiles of precision guided munitions, or must reconsider its commitments under the Oslo Convention prohibiting the use of cheaper, but highly effective ammunitions natures such as cluster munitions, used by both Russia and the U.S."

The Oslo Convention on Cluster Munitions prohibiting the use, transfer and stockpiling of cluster bombs was signed by over 100 states, including the U.K., in 2008. Both Russia and the U.S. did not sign the treaty.

Asked for a response to the RUSI report, a spokesperson for the U.K.'s Ministry of Defense told CNBC that "as the largest NATO defence spender in Europe, the U.K.'s armed forces are well equipped to take a leading role in countering threats and ensuring the safety and security of British people at home and abroad."

The MoD added that "the U.K. does not stand alone but alongside its NATO Allies, who work closely together across air, sea, land, nuclear and cyber to deter threats and respond to crises."

NATO members have previously agreed to spend 2% of their GDP on defense. But NATO estimates for 2019, released in June, show that only the U.S., U.K., Greece, Estonia, Romania, Poland and Latvia have met or surpassed the 2% target. NATO member Greece spent 2.24% of its GDP on defense, according to the data, while the U.K. and Estonia jointly held the second highest defense spend of a European NATO member, at 2.13% of GDP.