The chancellor is understood to be pressing Theresa May to make the key concession, as part of negotiations on the prime minister’s hopes for a cash boost for education before she leaves No 10.
MPs will vote later on a move by Dominic Grieve, the former attorney general, to require the Commons to be sitting in the run-up to the 31 October Brexit deadline, by amending Northern Ireland legislation.
However, other amendments could yet come forward in what is likely to be a day of frantic negotiations within the Tory party.
The campaign to stop Mr Johnson, if he wins the Tory leadership race, proroguing parliament to prevent MPs blocking a no-deal received a further boost from William Hague.
The former party leader said it “ought to be unthinkable that we could leave the EU by a manoeuvre, by a procedural ruse of some kind.”.
Mr Hague said parliament should “have its say”, saying of Mr Johnson’s flirting with proroguing parliament: “He should rule it out.”
The Independent has been told there are “ongoing discussions” at the top of the government about granting free votes, in response to MPs’ attempts to rein in Mr Johnson.
Mr Grieve’s amendment would require the government to update MPs every fortnight on progress being made in restoring the Stormont assembly through September and October.
And, crucially, ministers would have to table an approval motion – which could then be amended by MPs to try to prevent the UK crashing out of the EU without a deal.
It was the “hook” of amendable motions that allowed a cross-party group of MPs to pass legislation to force Ms May to seek an Article 50 extension to avoid a crash-out departure in March.
A free vote would lift the pressure on ministers who have spoken out against a no-deal to make a stand – even at the cost of their jobs.
They include David Lidington, the de-facto deputy prime minister, David Gauke, the justice secretary, Amber Rudd, the work and pensions secretary, and Greg Clark, the business secretary, as well as Mr Hammond.
Meanwhile, a £3bn funding boost for England’s schools is expected within days – but only after the chancellor blocked Ms May’s plan for a three-year cash injection costing up to £27bn.
Mr Hammond had indicated he was prepared to quit over the prime minister’s pitch for him to open his cheque book, as part of her hopes for a “legacy”.
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