Rishi Sunak’s plans to revive the stalled Rwanda asylum scheme do not go far enough, a panel of lawyers convened by a campaign group of Tory MPs has said.

In a blow to the Prime Minister, the so-called “star chamber” of lawyers said the emergency legislation, which MPs will vote on in Parliament on Tuesday, “provides a partial and incomplete solution” to the problem of asylum seekers using the courts to avoid being put on a plane to the African nation.

The lawyers, led by veteran Tory MP Sir Bill Cash, said “very significant amendments” to the legislation are needed.

“The Prime Minister may well be right when he claims that this is the ‘toughest piece of migration legislation ever put forward by a UK Government’, but we do not believe that it goes far enough to deliver the policy as intended,” they said in a summary of their opinion.

In a rare move intended to win over critics, the Government produced a summary of its own legal advice in support of the scheme on Monday.

The European Research Group (ERG) panel of lawyers – Sir Bill, former Brexit minister David Jones, Martin Howe KC and Barnabas Reynolds – suggested that some of the changes needed to tighten up the legislation may fall outside the scope of the Safety of Rwanda (Asylum and Immigration) Bill.

Their findings were presented at a meeting of the ERG and other groups on the Conservative right in Parliament.

There was no decision on how the groups should vote on Tuesday, although the ERG is set to have a further meeting on Monday evening, where former immigration minister Robert Jenrick – who resigned in protest at the legislation – will address them.

Mr Sunak has tried to find a middle ground in response to the Supreme Court ruling that his plan to send asylum seekers who arrive in the UK on small boats is unlawful.

His Bill allows ministers to disapply the Human Rights Act but does not go as far as overriding the European Convention on Human Rights.

The star chamber warned that the legislation would not block personal claims against being sent to Rwanda.

The lawyers also complained that the Bill’s plan to disapply elements of human rights law was “very narrow” and they argued it did not go far enough to address the risk of European judges blocking the plan.

“Were the Strasbourg court to decide that Rwanda is unsafe in general, it is unclear whether the Bill would be effective such that UK courts would be required to ignore that judgment,” the lawyers said.

Former home secretary Suella Braverman was among the MPs from the Tory right gathered in Parliament’s Portcullis House to hear the star chamber’s verdict.

Following the meeting which included five backbench factions – the ERG, the Conservative Growth Group, the New Conservatives, the Northern Research Group and the Common Sense Group – Mr Sunak was urged to withdraw his legislation.

ERG chairman Mark Francois said it was not “fit for purpose”, telling reporters: “The Government would be best advised to pull the Bill and come up with a revised version that works better than this one which has so many holes in it.”

But in a sign of the difficulties facing Mr Sunak in winning over Tory rebels, the moderate One Nation Conservatives will hold a separate evening meeting in Parliament to discuss whether the Prime Minister’s plan goes too far.

Mr Sunak has been unable to focus fully on seeking to unite his fractured party as he spent Monday giving evidence to the UK Covid-19 Inquiry about his time as chancellor.

But top ministers, including Foreign Secretary Lord Cameron, have been engaging with MPs over the weekend in order to quell any rebellion over the £290 million Rwanda plan.

Defence Secretary Grant Shapps insisted it would prevent the vast majority of attempts to use the courts to avoid being sent to the African nation.

He said the Government’s current assessment is that only one in 200 cases will pass through once the Bill becomes law, following claims the analysis dated back to March.

But critics of the plan dismissed the Home Office’s modelling of how effective it would be.

A senior Tory source said: “This is an outdated and analytically flawed model – from March – which came before defeats in the Court of Appeal and Supreme Court. Number 10 don’t realise the world has changed and that’s their fundamental problem.

“There was never any modelling done for the new Rwanda Bill because they failed to plan. Even this old, optimistic model says it could take more than two months to remove a migrant. It would be laughable if it wasn’t so serious.”

The Bill will have its first Commons test on Tuesday at the second reading stage – no Government legislation since 1986 has fallen at this hurdle.

But with Labour and other opposition parties expected to vote against it, a revolt by 28 MPs could wipe out the Conservative majority.

Tories with concerns could instead choose to abstain or back the legislation at this stage but then seek to toughen it up – or water it down, depending on their views – as it progresses through the Commons.

The Home Office published a summary of the Government’s legal position on Monday, with an evidence pack expected on Tuesday setting out the situation in Rwanda.

The summary document concludes: “This is a novel and contentious policy, and the UK and Rwanda are the first countries in the world to enact it together.

“There are risks inherent in such an innovative approach but there is a clear lawful basis on which a responsible government may proceed.”

Mr Shapps said he is “sure” it will get through the Commons, but acknowledged it could face trouble in the Lords, where the Government lacks a majority and cannot control the timetable in the same way.

The Government has insisted the Rwanda scheme, through which asylum seekers in the UK would be deported to Kigali, is a key part of Mr Sunak’s plan to “stop the boats” by acting as a deterrent to people seeking to cross the Channel.

But the Home Office has earmarked at least £700 million to manage the arrival of migrants on small boats until 2030, with the option of extending the contracts until 2034, according to commercial plans highlighted by the BBC.

The money will be spent running the Western Jet Foil facility in Dover and the reception centre at the former Manston airfield in Kent.