Britain will not be able to negotiate a meaningful deal with the EU by the time of the key parliamentary votes on Brexit this autumn, putting the onus on MPs to tell the government they will not accept a “blind Brexit”, Lord Mandelson, the former EU trade commissioner, has said.

He predicted MPs would demand the government seek the EU’s agreement for the UK to remain in the EU to give time for further talks beyond the March 2019 deadline.

He admittted the request for more negotiating time may be rejected by the EU, forcing the UK out of the EU without any clear agreement in place.

Mandelson, one of the main remain strategists, said the true crisis for the government was likely to come in the autumn if it emerged that the Conservatives’ internal fault lines meant the party had not found the time or means to negotiate a deal, especially over the prevention of a hard border in Ireland.

The EU wants a deal settled by the October EU heads of government meeting so as to give time for the agreement to be ratified by national parliaments ahead of the UK’s planned formal departure in March next year, the deadline for the two-year negotiating period allowed in the Article 50 negotiating process.

Speaking at a meeting of the Labour centre-left group Progress, Mandelson said: “The UK government is not going to be able to complete this process in the prescribed two-year period, and Britain needs to ask, and I hope the EU 27 can agree, to stop the clock and allow more time to have a serious thrashing out of the issues and a proper confrontation of the trade offs.”

He said Theresa May had dug herself into a deeper hole with a set of red lines that had made it impossible for the UK to reciprocate to concessions made by the EU. He said: “It now means the UK has run out of time to dig Britain’s negotiating position out of the hole, which is why we need to ask for time-out, suspension of the timetable and more time for further scales to fall from more people’s eyes, including in the cabinet, about the colossal economic price we will pay if we continue on the current course.”

He said there was no majority in the Commons for a no-deal option in which the UK left with no agreement on its future economic relationship with the EU. He admitted many EU leaders will resist an extension of the article 50 deadline of March 2019 partly because the European parliamentary elections next spring are due to be held without UK involvement.

He said: “the EU would love the UK to change its mind, or meet it half way, but now they now feel the best thing is to have done with it, to get it done with, so they can get on with their lives. So they would like a smooth glide path of the UK out of the EU, if that is going to remain the settled view of the British people.”

“Therefore to postpone the date of leaving is not attractive to them,” he admitted.

It has been argued that, as a compromise, the EU and UK could agree a blind Brexit in which the two sides agree a two-year standstill transition in the autumn, but only sign a limited heads of agreement very broadly setting out the future economic relationship between the EU and the UK. Mandelson argued that if the UK left on those terms, the UK would be negotiating its future trading relationship with the EU from a weaker bargaining position outside the EU.

But he remained optimistic that British public opinion was beginning to shift towards a rethink as the sheer complexity of Brexit was becoming ever more clear. He said: “Sometimes I wonder whether Brexit is going to defeat Brexit.”

Mandelson, who is reviled in sections of the modern Labour party, insisted he was agitating about Brexit not to undermine Corbyn but because he cared about the future of the UK.

At the same time he said: “Labour did not yet have a programme, or the planks of a manifesto, coming together that people will vote for at the moment. We have to engage in a process of public inquiry, innovation and rebuilding, which is all too absent in our party. There is a policy vacuum in the party.”

Supporters of Corbyn believe his manifesto in the 2017 election was one of the sources of his stronger than expected election showing.