David Cameron should urgently set up a royal commission to consider all the alternatives to Britain's failing drug laws, including decriminalisation and legalisation, an influential cross-party group of MPs has concluded.

The Commons home affairs select committee says after taking evidence from all sides of the drug debate, including from Russell Brand and Richard Branson, that "now, more than ever" there is a case for a fundamental review of all UK drug policy. "This is a critical, now-or-never moment for serious reform," they say.

Among the recommendations contained in a report published on Monday, the MPs say Home Office and health ministers should be sent to Portugal to examine its system of replacing criminal penalties for drug use with a new emphasis on treatment. They say the Portuguese example clearly reduced public concern about drug use and was backed by all political parties and the police.

The MPs also suggest the British government should fund a detailed research project monitoring the recent legalisation of marijuana in the American states of Washington and Colorado and the proposed state monopoly of cannabis production and sale in Uruguay.

The committee visited Colombia, the US and Portugal as part of their year-long inquiry.

In the report the MPs say: "We recommend the establishment of a royal commission to consider the best ways of reducing the harm caused by drugs in an increasingly globalised world.

"In order to avoid an overly long, overly expensive review process, we recommend that such a commission be set up immediately and be required to report by 2015."

Ministers should at the same time instigate a public debate in Britain on all the alternatives to current drug policy as part of the royal commission, recommends the report, Drugs: Breaking the Cycle. The government also needs to initiate a discussion within the United Nations' commission on narcotic drugs, including the possibility of legalisation and regulation – to tackle and reduce the harms from the global drug trade at home and abroad.

Government sources were dismissive of the move to a royal commission, insisting that drug use in Britain was at the lowest level since records began: "Our current laws draw on the best available evidence and as such we have no intention of downgrading or declassifying cannabis.

"A royal commission on drugs is simply not necessary. Our cross-government approach is working … We will respond more fully to the report in due course."

But the committee's chairman, the former Labour minister Keith Vaz, said that was no longer sustainable: "After a year scrutinising UK drugs policy, it is clear to us that many aspects of it are simply not working and it needs to be fully reviewed. We cannot afford to kick this issue into the long grass. We have recommended that a royal commission be set up with an end-date of 2015."

He said drugs cost thousands of lives and billions of taxpayer pounds annually: "This is a critical, now-or-never moment for serious reform. If we do not act now, future generations will be crippled by the social and financial burden of addiction."

Although ministers moved immediately to dismiss the call for a royal commission, the hard-hitting report from one of parliament's most influential select committees is indicative of a growing consensus at Westminster that reform of the 1971 Misuse of Drugs Act is long overdue.

It follows a six-year study by the UK Drugs Policy Commission that called for decriminalisation of possession of small amounts of illicit drugs and a new forum for all three main party leaders to reach a consensus on reform.

The report's recommendations echo many of the findings of the committee's last major inquiry into drugs in 2002, when Cameron was a member, which called for a less punitive approach to cannabis and ecstasy.

The new report "regrets" the government's decision in 2008 to toughen the law on cannabis possession but only on the chairman's casting vote after the issue split the committee. The rest of the report was endorsed by a majority of the MPs, with the Conservative Mark Reckless voting with Labour and Liberal Democrats while his Tory colleagues dissented.

The call for a royal commission, which has been a longstanding Liberal Democrat policy, was also welcomed by experts. Martin Barnes of Drugscope, the leading independent information centre on drugs, said that the debate had been too often clouded by polarised positions, partial evidence and anecdote.

"This is a situation that has not been helped when policy-makers and politicians are fearful of being accused of being 'soft' on drugs or their views and intentions distorted," said Barnes.

"A royal commission, with a clear timetable, would help break this impasse – but it will require robust terms of reference and a credible membership. There is already a substantial body of argument and evidence on reforming drug policy – including the recent report by the UK Drug Policy Commission – so any commission will need authority and momentum behind it to achieve change."

The detailed findings of the 151-page report say that the 2002 report's recommendation for a reassessment of the Misuse of Drugs Act was rejected by ministers on the grounds it conflicted with Britain's international obligations to curbing the global drug trade: "The message from Colombia and other supplier and transit states is clear – what the international community is currently doing is not working," says the report. "We are not suggesting that the UK should act unilaterally in these matters, but our government's position must be informed by a thorough understanding of the global situation and possible alternative policies."

The MPs' report also makes detailed recommendations to "break the cycle" of drug addiction, including improved treatment in prisons and the community and for early intervention with better education and preventive work. On the recent explosion of "legal highs" or new psychoactive substances, the MPs say that retailers, including "head shops" and online sellers should be liable for the harms caused by the untested substances they have sold, including for related deaths.

The committee says that Britain's approach to the laundering of drug money has also been far too weak and new legislation is needed to extend the personal liability of senior bankers found to be involved.

The MPs make detailed recommendations to combat the supply of drugs in prisons, including mandatory drug testing on arrival and release, and a new drive for recovery-based treatment behind bars.

On the drug treatment programme in the community said more use should be made of residential treatment and buprenorphine as alternatives to the widespread use of methadone as a heroin substitute.