The "dangerously weak" Royal Navy will put trade routes at risk from pirates and terrorists unless the Government buys more frigates, a think tank has warned.

The Royal United Services Institute said ships in the current fleet were near the end of their useful life and at least 10 more frigates were needed.

The warning comes as the Ministry of Defence (MoD) reviews its future military requirements and looks to cut costs as part of the Government's drive to tackle the budget deficit.

Devonport naval base, which is likely to come under close scrutiny during the review, is home to a flotilla of 14 frigates and destroyers which spend months at a time battling pirates and drug smugglers.

The article said the average age of surface combatant ships was forecast to rise from 15 years in 2012 to 21 years in 2021. HMS Chatham, one of Plymouth's 11 frigates, has been in service for 23 years.

In their article, Vice-Admiral Sir Jeremy Blackham and Professor Gwyn Prins argue a strong navy deters pirates, terrorists and non-friendly governments from disrupting trade routes by quietly patrolling the seas.

It points out that 95 per cent of British trade by volume and 90 per cent by value is carried out by sea.

But the Navy's policing role risks being undermined if it does not receive extra funding, they argue. The article adds that it would be a "grave failure" if the review "attended principally – or worse, exclusively" to the financial squeeze from the Government and political pressure over Afghanistan.

The article states: "No one associates the full supermarket shelves, the availability of a range of other goods, and the supply of fuels to power our homes, cars and industry with the free flow of sea trade.

"The free flow that makes globalised trade and the creation of prosperity possible depends prominently upon the presence of naval units at sea, unseen and silent and therefore easily forgotten."

The article argues that future orders should be "seriously cost-constrained" so that ships were more basic and more could be bought. The MoD has said one of its long-term aims will be to use less specialised, cheaper ships that are easier to sell abroad.

"Real world tasks urgently require significantly more surface combatants, of lower cost and capability. Use of the sea demands presence along the sea routes," the article said.

"A poised force is the prerequisite for preemptive action. It is also the prerequisite for surprise.

"Surprise is often the ability to appear without warning and in force.

"The ships needed to fulfil these missions must have endurance, versatility, role adaptability and number, and be cheaper."

The Government has said only maintaining the Trident nuclear deterrent will be spared from the defence review.

A spokesman for the MoD said: "The future configuration of our armed forces will be based on the findings of the Strategic Defence and Security Review (SDSR) which is under way.

"The complex process of a SDSR will conclude in the autumn and speculation at this stage about its outcome is entirely unfounded."

Speaking on BBC Radio 4's Today programme yesterday, Prof Prins said frigates were the "glue" that held the service together.

"They need to be there because they provide mass and they provide presence," he said.

"What we are lacking is the essential glue that makes the Navy coherent."

But appearing alongside him, former naval officer Lewis Page said it was a mistake for the service to be "institutionally wedded" to buying more frigates.

"The lesson of World War II was big service combatants, battleships, had been replaced by aircraft carriers," he said.

"Nowadays we have helicopters, and whenever a frigate does anything useful, which is quite rare, it is usually its helicopter that did it," he continued