Theresa May could be forced to stand down if her Brexit deal is defeated in the Commons next week, a Tory former leader has warned, as a cabinet minister broke ranks to suggest the Norway-plus model as a viable “plan B”.

Eurosceptic Iain Duncan Smith cautioned against May and her cabinet deciding to “brazen it out”, saying such an approach would be a “disaster”.

“How the PM responds after the vote matters more than anything else she has done,” he told the Daily Telegraph (£). “I believe that if the response is, ‘we’ve lost but we will do this all over again’ it will become a leadership issue. I don’t want it to be. If she and the cabinet decide to brazen it out and simply say [a defeat of] anything under 200 is not as big as you think, then that would be a disaster.”

The paper reported cabinet ministers have also warned May she would have to stand down if the deal is defeated and she fails to secure better terms from the European Union.

The Times reported on wider unrest among MPs, saying Labour MPs were seeking an alliance with disaffected Tories and the Democratic Unionist party to bring about a leadership vote (£) in the event the deal is voted down.

Amber Rudd, the work and pensions secretary, made the unusual move of publicly airing a “plan B” (£) if May’s deal faltered, proffering the Norway-plus option. She said she still backed May and her vision of Brexit, but told the paper the Norway option “seems plausible not just in terms of the country but in terms of where the MPs are”. However, she added “nobody knows if it can be done”.

It came as the Labour leader, Jeremy Corbyn, said he would consider delaying Britain’s exit from the EU to negotiate a better deal if his party came to power. “If we go into government straight away we would start negotiating straight away. If it meant holding things a bit longer to do it, of course,” he told Sky News.

Corbyn said his party was ready to “step in and negotiate” with the EU, and would form a minority government “if that is what is on offer”.

On Friday, a government assessment warned a no-deal Brexit could lead to six months of chaos on key cross-Channel routes, with ferries between Dover and Calais and Channel Tunnel traffic facing disruption until the end of September 2019.

A letter sent by the health secretary, Matt Hancock, (pdf) to the pharmaceutical industry and NHS said: “The revised cross-government planning assumptions show that there will be significantly reduced access across the short straits, for up to six months.

“This is very much a worst-case scenario; however, as a responsible government, we have a duty to plan for all scenarios.” Ministers are drawing up plans to fly in vital drugs and give priority to lorries carrying medical supplies at gridlocked ports.

Kent county council’s leader, Paul Carter, called for emergency measures to prevent lorries entering the county to avoid chaos on the roads. “We now need far more input and information from national government in how they are going to work with us,” he said.

With the Commons vote on a withdrawal agreement expected to result in it being rejected by MPs, the risk of a no-deal Brexit could increase.

Pressure is mounting on May to delay the 11 December vote to give herself time to ask for more concessions from the EU at a Brussels summit at the end of next week, with 29 Tories saying they would vote against the deal, analysis showed.