Boris Johnson is facing a backlash from his own MPs after his government dismissed demands for new laws to protect the rights of EU citizens in the event of a no-deal Brexit.

Conservative MPs warned that failure to guarantee the rights in law would be an “abrogation of responsibility”, because the EU is unlikely to guarantee the rights of the 1.3 million Brits living in its member states unless the UK acts first.

But former Conservative leader Iain Duncan Smith, a senior ally of Mr Johnson, told The Independent that ministers would be reluctant to bring forward a news law on citizens’ rights because of fears that it would be “hijacked” by MPs trying to block a no-deal Brexit.

The claim came as Tory MPs warned Mr Johnson not to prioritise a no-deal exit ahead of guaranteeing the rights of 3 million EU citizens living in the UK, and Brits living in the EU.

MPs said that some Tories would be willing to support Mr Johnson’s threat to opt for a no-deal Brexit if citizens’ rights were protected, but would not contemplate doing so otherwise – potentially stopping the new prime minister from getting a no-deal plan through parliament.

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Speaking in the House of Commons on Thursday, Mr Johnson said he wanted to “repeat unequivocally our guarantee to the 3.2 million EU nationals now living and working among us”.

He added: “I thank them for their contribution to our society and for their patience. I can assure them that under this government they will have the absolute certainty of the right to live and remain.”

However, he faced an angry backlash minutes later when his spokesman admitted that Mr Johnson was not talking about new legislation to protect EU nationals’ rights.

Instead, the government believes that the existing Home Office scheme offering EU citizens “settled status” is enough to guarantee their rights.

But critics said the settled status scheme could be scrapped or changed “on a whim” and provided no security for EU citizens.

They also said the government must act to help EU nationals in order to protect the 1.3 million British citizens living in EU countries. Some EU member states, including Spain, have said they will only guarantee the rights of British nationals in their country if the UK government acts to protect their nationals living in the UK.

As the row over the issue deepened, Mr Duncan Smith suggested that the new government would not legislate to help EU citizens because ministers fear the draft law would be “hijacked” by MPs determined to stop Brexit or avert a no-deal outcome.

Mr Duncan Smith told The Independent: “Citizens’ rights will be guaranteed categorically but we don’t want legislation because that opens the door to all sorts of Christmas tree amendments.

“We don’t want legislation in the next period because that legislation will simply get hijacked and without a big majority it’s going to be difficult to stop it.

“If the door opens, [anti-Brexit MPs] will pile in with these amendments and lots of them will be aimed at bogging this thing down.”

He suggested that legislation guaranteeing citizens’ rights could be passed within a week of Britain leaving the EU on 31 October but warned that doing so before then would lead to “all sorts of trouble”.

He said: “Citizens’ rights are not something you can deal with before you leave. If you leave and immediately legislate then the issue would be solved within a matter of days. It’s just a tactical fact that doing so before leaving risks creating a nightmare.”

No 10 refused to comment on Mr Duncan Smith’s claim that the government was reluctant to table new legislation for fear of it being used to block no deal. A Downing Street source said: “Any plans for legislation will be announced in the usual way and with relevant notice to parliament. The commitment to EU citizens shouldn’t be used as a political football.”

But some Tory MPs warned that the matter would become a major “toxic” issue for the party if the rights of EU nationals were not enshrined in law.

Conservative MP Alberto Costa, who has led the campaign to guarantee the rights of EU citizens and British citizens in the EU, told The Independent: “The new prime minister has repeatedly promised and pledged to absolutely, unequivocally guarantee the rights of EU nationals. The way to do that is through the law.

“If anyone intended it to mean something else, then I don’t know which form of democracy we are referring to. I await the proposed law that will absolutely unequivocally guarantee the rights of EU nationals.”

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In a clear warning to Mr Johnson, he continued: “If EU nationals’ rights are not unequivocally, absolutely guaranteed as has been repeatedly promised, it will be one of the biggest toxic issues for the Conservative Party.

“It is imperative for the Conservative Party to do the morally right thing in the event of no-deal and that is to uphold the rights of 3 million innocent people who were disenfranchised from the EU referendum.

“That comes first and I would expect every MP in the House of Commons to have that at the forefront of their minds … If the rights of EU nationals are not enshrined in law, I cannot see how they can be protected.”

He insisted that the current settled status scheme was not sufficient because: “It’s created through Home Office rules and Home Office rules can be changed at the whim of the home secretary. That is completely different to the rights that EU nationals enjoy right now – treaty rights underpinned by international law.”

Mr Costa said that protecting the rights of EU nationals was also crucial in forcing European states to guarantee the rights UK citizens living in EU countries. Failure to do so would mean the UK was “abrogating its responsibility to 1.3 million British citizens who rely on those treaty rights in order to subsist, to work, to live, to retire, to access healthcare, to get pension entitlements, to get benefits”.

He said: “We have never seen a peacetime government abrogating its responsibility to over 1.3 million of its own citizens.”

Mr Costa also hit back at Mr Duncan Smith, saying of Brexiteers who insist that legislation is not needed: “They’re putting at risk British citizens and their rights. They’re being glib about those rights and how we protect them.”

He added: “It is the first duty of any government and of members of parliament to protect British citizens and defend their country. The only way of protecting British citizens in the event of no deal is to protect EU nationals here first.

“That has got to be clearly understood by Brexiteers. My priority is the protection of British citizens wherever they may be and I would have thought that that must be the number one priority of any MP regardless of their view on Brexit.

“They cannot have it both ways. They cannot say on the one hand that they will protect EU rights and on the other say that we don’t need any legislation to do so. You either guarantee those rights or you don’t.”

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During the 2016 referendum campaign, Mr Johnson was among Vote Leave campaigners who signed a pledge saying there should be “no change” in the rights of EU citizens in the UK after Brexit. It said: “EU citizens will automatically be granted indefinite leave to remain in the UK and will be treated no less favourably than they are at present.”

Critics say the settled status scheme does not meet this test because EU nationals are not “automatically” granted the right to remain in the country and because, without a Brexit deal, their rights will not be enshrined in law once Britain leaves the EU.

Mr Costa said: “That pledge is not honoured if there is no law passed before 31 October. It’s a breached promise to the electorate.”

The Independent understands that a group of MPs from across the Commons travelled to Brussels last week to meet with Michel Barnier, the EU’s chief Brexit negotiator, to discuss the issue of citizens’ rights.

The delegation included Mr Costa, Tory Brexiteer Sir Edward Leigh, Conservative peer Viscount Ridley, Labour’s Baroness Hayter, Liberal Democrat Layla Moran and the SNP’s Stuart McDonald. The group emphasised that there was cross-party support in parliament for the reciprocal rights of citizens to be guaranteed even if there was a no-deal Brexit.

One source said the Remain-backing members of the group had made clear to Mr Barnier “that they cannot rule out no-deal, the spectre of no deal is there, and that he has got to take seriously reopening the withdrawal agreement and carving out citizens’ rights in the event of no-deal”.