Brexit will have to be delayed unless a deal is struck by the end of the year because the European parliament will run out of time to vote on it, MPs have been told.

Guy Verhofstadt, the parliament’s Brexit coordinator, said it needed to begin scrutinising any agreement at least three months before exit day – which is 29 March next year.

Asked to give the deadline for a deal, he replied: “Before the end of the year. That is the ultimate date, otherwise we cannot be assured to have a vote in plenary in March.”

Mr Verhofstadt said he remained optimistic of a deal in the autumn – despite the UK and EU remaining far apart on key issues – but warned: “You cannot go beyond 2018.”

The comment came as the former Belgian prime minister dismissed both of the solutions put forward by Theresa May to solve the impasse over customs and the Irish border.

On the prime minister’s preferred “customs partnership” – under which the UK would collect EU duties – he said: “No, we cannot outsource our competences on customs duties. That will not happen.”

And he warned the rival, technology-based “max fac” idea could bring the return of violence to Ireland, by requiring cameras and a border, calling it “a dangerous step to do”.

Hopes of a Brexit deal by October have all but evaporated with the negotiations deadlocked over customs, Ireland and European court oversight.

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

Mr Verhofstadt refused to speculate on whether a failure to agree a deal by December’s EU summit would trigger a request to extend the Article 50 deadline of March next year.

It was “not a good moment” to speculate because that would “take pressure away from those who have to deliver in October or November”.

Giving evidence to the Commons Brexit Committee, the EU parliament’s negotiator also:

* Branded the UK’s plan for a one-year customs “backstop” to solve the Irish border issue “not acceptable” – because it could not be time limited.

* Dismissed David Davis’s claim that the thrust of a future trade deal could be agreed by next March – insisting it would take “the whole transition period”, until the end of 2020.

* Insisted it was “impossible” for the UK to stay within the European arrest warrant – arguing the best hope was an extradition agreement that retained its benefits as “far as possible”.

* Expressed hope of a last-gasp UK change of mind to stay in the customs union and single market, saying: “I think there is no opposition from the European side if that proposal comes to Brussels.”

* Said Brexit had “opened our eyes in the European Union” about the need for reform – but denied that necessarily meant more integration, which the UK would have opposed.

* Argued the UK should pursue an “association agreement”, along the lines of Ukraine, which does not require ratification by all EU nations but “can take some time”.