Keir Starmer has warned Boris Johnson that MPs will “do everything to stand in his way” if he tries to force through a “bad deal or a no-deal Brexit”.

Johnson, the frontrunner in the race to be Britain’s next prime minister, has suggested he will “disaggregate” Theresa May’s “otherwise defunct” withdrawal agreement and implement its less contentious elements.

But research commissioned by Starmer, the shadow Brexit secretary, from the House of Commons library suggests this would still require the government to bring legislation before parliament, which MPs could then reject or amend.

The library’s experts said: “Without an act of parliament, the UK cannot ratify the withdrawal agreement.”

They added that aside from “undertakings” the government gave that did not require changes to the law, “the UK cannot in any meaningful sense ‘implement’ individual parts of the withdrawal agreement otherwise than by primary legislation”.

Starmer said: “Boris Johnson’s Brexit plan is a non-starter. If he tries to force through a bad deal or a no-deal Brexit then parliament will do everything it can to stand in his way.”

The chancellor, Philip Hammond, earlier this week became the latest Conservative MP to signal that he could vote to block a no-deal Brexit, and will join a growing band of rebels on the Tory backbenches if he is sacked by the new prime minister as expected.

Starmer said: “Labour will work with all sides, even former members of Theresa May’s cabinet, to protect the country from a no-deal Brexit.”

Johnson’s team believe parliament would be unable to block a no-deal Brexit on 31 October because it remains the legal default.

Q&A What does a no-deal or WTO-rules Brexit mean? Show Hide If the UK leaves the EU without a deal it would by default, become a “third country”, with no overarching post-Brexit plan in place and no transition period. The UK would no longer be paying into the EU budget, nor would it hand over the £39bn divorce payment. The UK would drop out of countless arrangements, pacts and treaties, covering everything from tariffs to the movement of people, foodstuffs, other goods and data, to numerous specific deals on things such as aviation, and policing and security. Without an overall withdrawal agreement each element would need to be agreed. In the immediate aftermath, without a deal the UK would trade with the EU on the default terms of the World Trade Organization (WTO), including tariffs on agricultural goods. The UK government has already indicated that it will set low or no tariffs on goods coming into the country. This would lower the price of imports – making it harder for British manufacturers to compete with foreign goods. If the UK sets the tariffs to zero on goods coming in from the EU, under WTO “most favoured nation” rules it must also offer the same zero tariffs to other countries.

WTO rules only cover goods – they do not apply to financial services, a significant part of the UK’s economy. Trading under WTO rules will also require border checks, which could cause delays at ports, and a severe challenge to the peace process in Ireland without alternative arrangements in place to avoid a hard border.

Some no-deal supporters have claimed that the UK can use article XXIV of the General Agreement on Tariffs and Trade (Gatt) to force the EU to accept a period of up to 10 years where there are no tariffs while a free trade agreement is negotiated. However, the UK cannot invoke article XXIV unilaterally – the EU would have to agree to it. In previous cases where the article has been used, the two sides had a deal in place, and it has never been used to replicate something of the scale and complexity of the EU and the UK’s trading relationship. The director general of the WTO, Roberto Azevêdo, has told Prospect magazine that “in simple factual terms in this scenario, you could expect to see the application of tariffs between the UK and EU where currently there are none”. Until some agreements are in place, a no-deal scenario will place extra overheads on UK businesses – eg the current government advice is that all drivers, including lorries and commercial vehicles, will require extra documentation to be able to drive in Europeif there is no deal. Those arguing for a “managed” no deal envisage that a range of smaller, sector-by-sector, bilateral agreements could be quickly put into place as mutual self-interest between the UK and EU to avoid introducing or to rapidly remove this kind of bureaucracy. Martin Belam

But separate analysis by Labour suggests a string of emergency bills on issues from medicines regulation to new rules and systems for the border in Northern Ireland would still need to be passed, even in a no-deal scenario.

Any of them could potentially be seized on – and amended – by MPs looking for a mechanism to block a no-deal Brexit, or perhaps even revoke article 50.

Starmer said: “It’s simply unsustainable to sideline parliament on an issue of this importance. And it’s deeply irresponsible to ignore the legal black hole that a no-deal Brexit would cause.”

However, Johnson’s team insist they are likely to be able to persuade the EU27 to make changes to the withdrawal agreement, including the contentious Irish backstop.

They then believe Johnson will return in triumph to Westminster and win a majority for his Brexit deal in parliament.

Johnson hopes the threat to Tory seats from Nigel Farage’s Brexit party, combined with a perception that he – unlike May – would be willing to press ahead with a no-deal departure if MPs reject his agreement, will change the political mood.

This interpretation has been supported in recent days by several Labour MPs including Lisa Nandy and Gareth Snell suggesting they could vote for something similar to the withdrawal agreement if it were put before parliament.

Labour’s position, reiterated by Jeremy Corbyn in the Commons on Wednesday, is that any Brexit deal should be subject to a referendum.

He asked May: “Does the prime minister agree that whoever succeeds her should have the courage to go back to the people with their preferred Brexit option to end the uncertainty and get Brexit resolved?”

May responded by criticising Labour’s refusal to support her Brexit deal.

Starmer is one of the shadow ministers, alongside the key Corbyn allies Diane Abbott and John McDonnell, who are pressing for Labour to declare it would support remain in any such referendum.

But Corbyn has insisted he is continuing to consult on the issue. And he has made reference to the precedent set by the former Labour prime minister Harold Wilson.

Wilson called the 1975 referendum on common market membership when he was presiding over a deeply divided Labour cabinet, but while the government’s official position was to recommend staying in, he allowed ministers to campaign on both sides.