The government has repeatedly failed to invest in defence and security, leaving the UK struggling to cope with emerging threats, an influential parliamentary committee was due to warn on Sunday in a report thrown into sharp relief by the developing crisis in the Persian Gulf.

The Joint Committee on the National Security Strategy says the next prime minister will have to tackle major challenges – from growing strains on the UK’s relationship with the US and the EU, to the rise of China – if the UK is to meet its defence ambitions.

“If the Government wants to turn the ‘Global Britain’ concept into a meaningful strategy for a positive and self-assured role for the UK after its departure from the EU, then it needs to be more honest about how it proposes to address these challenges,” said the committee chair, Margaret Beckett MP. “It must also back this up with the necessary funding and resources, especially for defence and diplomacy.”

The report’s publication comes after the former chief of defence staff, Lord Richards, warned that the Royal Navy was “just too small to have a significant effect without being with allies”.

Speaking in response to the seizure of a British-flagged oil tanker by Iranian authorities, Richards told BBC radio’s Today programme: “The fact is, for whatever reason, our government has invested relatively little against the threat and the risks that we face, particularly more and more in the longer term.”

The former first sea lord, Lord West of Spithead, told Sky News that the navy had “too few ships” to escort merchant vessels in dangerous waters.

Concerns about an overstretched military are echoed in the committee’s report, which warns that the UK’s defence funding model is broken and that the UK’s long-term plan for defence, established in 2015, is unaffordable within the budget allocated.

The government is accused of persistently failing to provide enough money to fund its ambitions for defence capabilities and the Ministry of Defence is criticised for struggling to manage its budget effectively.

“Strong arguments have been advanced that it is not enough to spend 2% of GDP on defence, in light of both the scale and range of threats to the UK and the costs involved in keeping pace with rapid technological change,” the report recounts. “Yet a recent Defence Committee report found that Ministry of Defence expenditure fell from 2.4% of GDP in 2010/11 to 1.8% in 2017/18.”

The report also suggests tensions between two key Whitehall departments need to be resolved if the UK is to meet its defence objectives.

“There is an urgent need for the MoD to be supported by the Treasury in its efforts to harness new technology and innovation while maintaining sufficient numbers of soldiers, sailors, pilots and more conventional equipment,” the report adds.

Instead it finds that too often policy and budgetary decisions are left hanging. “The national security landscape is changing more quickly than the current cycle of five-yearly reviews of UK national security can accommodate, suggesting that a form of interim review is needed.”