Labour is planning to force a Commons vote within weeks that would make it impossible for Britain to crash out of the European Union without a deal, as fears grow about a disastrous hard Brexit if parliament rejects Theresa May’s agreement.

Keir Starmer, the shadow Brexit secretary, is working on plans to amend key elements of Brexit legislation that still have to pass through the Commons, in order to prevent parliament ever approving the option.

He says he has had assurances from a number of Tory MPs who say they would help push his amendments through parliament to avoid chaos. It is understood that some of them are serving Conservative ministers.

With Labour, the 10 Democratic Unionist party MPs, and upwards of 40 Tory MPs ready to vote down May’s deal, senior Labour figures are aware that they need to avoid the possibility of being blamed for rejecting a Brexit agreement that was on the table and helping to trigger chaos that could lead to “no deal”.

Starmer told the Observer that Labour would make sure parliament offered a legislative route to make “no deal” impossible. “If the prime minister’s deal is rejected – and that’s looking increasingly likely – parliament will not just sit back and allow her to proceed with no deal,” he said.

“There are plenty of Conservative MPs who have come up to me to say that they will not countenance the UK crashing out of the EU without an agreement. There is a clear majority in parliament against no deal, and Labour will work across the Commons to prevent no deal. On the government’s own analysis, over 50 changes to legislation would be needed for a no-deal outcome, so there will be no shortage of opportunities to pass binding votes on this issue.”

Many Labour MPs in Leave constituencies had flirted with the idea of backing May’s deal on the grounds that their constituents want to see Brexit delivered, and that the alternatives of crashing out of Europe without an agreement or holding a second referendum would be unacceptable to many voters.

Labour whips are now confident, however, that very few – if any – Labour MPs will back May’s deal because they have concluded it would also be disastrous for their constituents, leaving Britain locked into the EU’s economic system for the foreseeable future but with no say in setting the rules that govern it. Two Labour MPs who had suggested they might back May’s deal, Gareth Snell and Ruth Smeeth, both said last week that they would not back it.

Analysis by the Institute for Government reveals that the Brexit department’s no-deal technical notices commit the government before next March to the creation or expansion of 15 quangos, further legislation in 51 areas and the negotiation of 40 new international agreements either with the EU or other countries.

Labour’s current policy is to push for a general election to resolve the Brexit deadlock, but only after May’s deal is voted down. If it fails it is keeping “all options open”, including a second referendum. A vote of no confidence requires a simple majority of the Commons to pass, although under the Fixed-term Parliaments Act, a 14-day period is allowed for a new government to be formed with the approval of MPs.

A debate is now under way at the top of the party over how soon Labour should table a motion of no confidence in the government. Some believe that anger among the DUP at the Brexit deal could open the way for the government to lose a confidence vote. At present, the DUP is relied upon to prop up May’s minority administration.

Some insiders believe the best moment to strike will be immediately after May loses a vote on her Brexit deal. Launching a no confidence motion against her would also fulfil Jeremy Corbyn’s pledge that he will seek a general election before backing other options, such as a second referendum.

Should the government lose a no confidence motion, Labour would have a 14-day period to attempt to form a new administration or an election is called. However, while some senior figures say the time for such a putsch is “getting close”, others are warning that it could backfire.

“Any false move by us will have possibly dire consequences if we misunderstand the mood in the country,” said one shadow cabinet member. “Any premature attempt by us which fails will look like either we want to overturn the referendum result or more likely that we see it as an opportunity to make an unprincipled dash to grab power.”

Meanwhile, Labour backers of a second referendum believe that the leadership is now facing a “tidal wave” of demands for the party to back the idea once May’s deal has been voted down.

“From every part of the party, unions, MPs and members, there are growing demands,” said one senior MP. “Huge parts of the parliamentary party now talk supportively of it, and it has to be where we end up.”