MPs are likely to reject the Brexit deal in the autumn unless an agreement on future trade with the EU has also been secured, David Davis has admitted.

Theresa May will have the “hardest time” convincing the Commons to sign up to the £39bn-plus “divorce bill” if only flimsy details of the long-term trade deal are known, the brexit secretary said.

“It has to be substantive from a British parliament point of view,” Mr Davis told MPs on the Brexit committee.

He pointed out that the Commons will also be “voting for a bill of £39bn”, adding: “It will want to know what it is getting in exchange.

“The hardest time I will have in October is people saying, ‘what have we got for this?.’”

The comments will be viewed as a possible hostage to fortune, because the EU has insisted there is only time to agree a “framework” for future trade by its October deadline.

During the evidence session, Mr Davis also:

* Insisted the government would win a critical looming Commons vote on leaving the EU customs union, saying: “I expect the government’s policy to be upheld.”

* Denied Brussels had rejected the UK’s technology-based proposals to avoid a hard border in Ireland – insisting that was merely an “opening negotiating position”.

* Set up a future clash over the backstop of “full alignment” of regulations across Ireland, if necessary – revealing the UK would put forward its “own wording” based on “mutual recognition” of standards.

* Twice failed to rule out going back to the negotiating table if MPs reject the exit deal and vote for further talks – despite previously insisting the UK would then crash out with no deal.

* Claimed the Treasury’s own studies, showing the UK will be worse off under every Brexit outcome, were “wrong”, saying: “The best modelling in the world is wrong.”

* Acknowledged the two-year Article 50 process could be extended to delay Brexit beyond March 2019 if there was “unanimity” among the other EU members.

Brexit threatens life on the Irish border: in pictures Show all 15 1 /15 Brexit threatens life on the Irish border: in pictures Brexit threatens life on the Irish border: in pictures An abandoned shop is seen in Mullan, Co Monaghan. The building was home to four families who left during the Troubles. The town was largely abandoned after the hard border was put in place during the conflict. Mullan has seen some regeneration in recent years, but faces an uncertain future with Brexit on the horizon Reuters Brexit threatens life on the Irish border: in pictures A defaced ‘Welcome to Northern Ireland’ sign stands on the border in Middletown, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Mervyn Johnson owns a garage in the border town of Pettigo, which straddles the counties of Donegal and Fermanagh. ‘I’ve been here since 1956, it was a bit of a problem for a few years. My premises has been blown up about six or seven times, we just kept building and starting again,’ Johnson said laughing. ‘We just got used to it [the hard border] really but now that it’s gone, we wouldn't like it back again’ Reuters Brexit threatens life on the Irish border: in pictures Farmer Gordon Crockett’s Coshquin farm straddles both Derry/Londonderry in the North and Donegal in the Republic. ‘At the minute there is no real problem, you can cross the border as free as you want. We could cross it six or eight times a day,’ said Crockett. ‘If there was any sort of obstruction it would slow down our work every day’ Reuters Brexit threatens life on the Irish border: in pictures John Murphy flies the European flag outside his home near the border village of Forkhill, Co Armagh Reuters Brexit threatens life on the Irish border: in pictures Potter Brenda McGinn stands outside her Mullan, Co Monaghan, studio – the former Jas Boylan shoe factory which was the main employer in the area until it shut down due to the Troubles. ‘When I came back, this would have been somewhere you would have driven through and have been quite sad. It was a decrepit looking village,’ said McGinn, whose Busy Bee Ceramics is one of a handful of enterprises restoring life to the community. ‘Now this is a revitalised, old hidden village’ Reuters Brexit threatens life on the Irish border: in pictures Union Flag colours painted on kerbstones and bus-stops along the border village of Newbuildings, Co Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures Grass reflected in Lattone Lough, which is split by the border between Cavan and Fermanagh, seen from near Ballinacor, Northern Ireland Reuters Brexit threatens life on the Irish border: in pictures Donegalman David McClintock sits in the Border Cafe in the village of Muff, which straddles Donegal and Derry/Londonderry Reuters Brexit threatens life on the Irish border: in pictures An old Irish phone box stands alongside a bus stop in the border town of Glaslough, Co Monaghan Reuters Brexit threatens life on the Irish border: in pictures Billboards are viewed from inside a disused customs hut in Carrickcarnon, Co Down, on the border with Co Louth in the Republic Reuters Brexit threatens life on the Irish border: in pictures Seamus McQuaid takes packages that locals on the Irish side of the border have delivered to his business, McQuaid Auto-Parts, to save money on postal fees, near the Co Fermanagh village of Newtownbutler. ‘I live in the south but the business is in the North,’ said McQaid. "I wholesale into the Republic of Ireland so if there’s duty, I’ll have to set up a company 200 yards up the road to sell to my customers. I’ll have to bring the same product in through Dublin instead of Belfast’ Reuters Brexit threatens life on the Irish border: in pictures A disused Great Northern Railway line and station that was for customs and excise on the border town of Glenfarne, Co Leitrim Reuters Brexit threatens life on the Irish border: in pictures Alice Mullen, from Monaghan in the Republic of Ireland, does her shopping at a former customs post on the border in Middletown, Co Armagh. ‘I’d be very worried if it was a hard border, I remember when people were divided. I would be very afraid of the threat to the peace process, it was a dreadful time to live through. Even to go to mass on a Sunday, you’d have to go through checkpoints. It is terribly stressful,’ said Mullen. ‘All those barricades and boundaries were pulled down. I see it as a huge big exercise of trust and I do believe everyone breathed a sigh of relief’ Reuters Brexit threatens life on the Irish border: in pictures A bus stop and red post box stand in the border town of Jonesborough, Co Armagh Reuters

The Independent revealed at the weekend that ministers now accepted the October motion could be amended – rather than being a “take-it-or-leave-it” choice.

Stephen Kinnock, a Labour MP, seized on Mr Davis’ confirmation of that, to suggest the country was on track for a potential “constitutional crisis”.

He asked if the government would accept an amendment to “go back to Brussels and negotiate a different deal” – which the brexit secretary did not rule out.

“I’m not going to speculate on amendments that have not even yet been laid, let alone been passed by the House,” he replied.

Mr Davis also denied suggestions the government was “winging it” over Brexit.

“No, we are not winging it but we are having to accommodate different changes as we go along,” he said.

And the brexit secretary insisted there would not be a repeat of the Windrush scandal, with the children of EU citizens living in Britain, when they are required to apply to stay in the UK.