COVID vaccine passports could be blocked in the Commons as Labour threatens to team up with Tory rebels to vote it down.
Shadow health secretary Jonathan Ashworth said the scheme risks being "discriminatory" and "unfair" and accused the PM of failing to be "honest" with people about how it will work.
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He branded the blueprint an effective "digital ID card" by the back door and said the fact shops and pubs could ask for proof of a jab as a condition of entry is a major concern.
The intervention will cause No 10 a major headache, with a large mutiny already brewing on the Conservative benches as ministers confirmed they will hold a parliamentary vote over the plan.
So far 41 Tories have put their names to a letter calling vaccine passports "divisive and discriminatory", putting the Government's majority of 87 in jeopardy and meaning it may need the support of opposition MPs.
Mr Ashworth accused ministers of being "at sixes and sevens" over the proposal and said the PM's evasive press conference performance last night showed he "couldn't explain his own policy".
He said: "You've now got the vaccine minister saying it's not for shops and pubs, the document published last night permits shops and pubs to ask you whether you've had a vaccination as a condition of entry.
"I do think it is discriminatory to say to somebody that you cannot go into Next or H&M unless you produce your vaccination status on an app, unless you produce that digital ID card.
"I don't think that is fair. Now if ministers are saying that is not what the policy is then they have to explain why does the policy document they produced last night permit that scenario?
"So, there's a lot of confusion out there. I just want ministers to be honest and straight with us and tell us exactly what their policy proposals are."
Labour sources said leader Sir Keir Starmer is set to oppose the Government's vaccine passport plans in their current form, and wants to focus on getting everyone vaccinated instead.
A party source told Politico: “On the basis of what we’ve seen and discussed with ministers, we oppose the government’s plans for domestic vaccine passports.
"They appear poorly thought through, will put added burdens on business and run the risk of becoming another expensive Whitehall project that gets outsourced to friends of Tory ministers.
"Labour’s focus would be on getting the vaccine out, fixing self-isolation and contact tracing.”
The move would see the left-wing party team up with Tory backbenchers and the Lib Dems to vote against the plan.
Mark Harper, chairman of the Covid Recovery Group, said: “Trying to introduce these domestic vaccine passports by the back door by linking them to removing social distancing rules just won’t be acceptable.
"Whether the state legislates for it, recommends it or simply allows it, Covid Status Certification will lead to a two-tier Britain and these issues need debating thoroughly and carefully."
Last night the Government published an update on its rolling review into the use of jabs certificates which said pubs may have to demand them to ditch ruinous social distancing rules.
The document said vaccine passports "may be required for a period after all adults have been offered a vaccine, in order to prevent a surge in hospitalisations which could put unsustainable pressure on the NHS".
And it made clear such documents will almost certainly be required to attend large scale events such as football matches, theatre performances, and music festivals.
It states: "The Government believes that COVID-status certification could have an important role to play both domestically and internationally, as a temporary measure.
"It is possible that COVID-status certification could also play a role in reducing social distancing requirements in other settings which people tend to visit more frequently, for example in hospitality settings.
"However, the Government recognises this has significant implications for businesses and their customers, so this will be further considered in consultation with industry."
But in his press conference remarks immediately afterwards the PM was much more vague, and tried to skirt the issue by insisting mass free testing will play a major part in reopening the economy.
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He said there were "complicated ethical and practical issues raised by the idea of Covid status certification using vaccination alone" and that "we want to be going ahead in the next few weeks with some test events".
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And today vaccines minister Nadhim Zahawi insisted no decisions have yet been made, stressing Brits won't need a vaccine passport to go to beer gardens from next Monday, or when pubs fully open on May 17.
He added: "But it's only responsible as we see how this virus behaves, as we see how other countries are utilising technology to make sure that they keep the virus under control, then we should look at the same thing."