The British Medical Association’s lawyers have warned the union that its planned legal action against the government will “definitely not” prevent the health secretary from imposing a contract on England’s 37,000 junior doctors, HSJ can reveal.

Although they said the planned judicial review had a better than even chance of success, the lawyers added that government could easily overcome this challenge and continue with the imposition.

The lawyers advised that there was nothing “inherently unlawful” about the government’s proposed contract, and the judicial review was likely to be unusually expensive and a considerable drain on the BMA’s resources.

They concluded that the judicial review should instead be seen as a “last throw of the dice” – with its main potential benefit being a new window of opportunity for the junior doctors’ committee to secure further improvements to the contract about to be imposed.

To do this, they suggested, the committee should consider keeping the judicial review secret for one to two weeks, to make it easier for health secretary Jeremy Hunt to reopen negotiations.

However, contrary to this advice, the BMA announced its plans to launch a judicial review – and three 48-hour strikes – on Tuesday last week. The written legal advice is dated 19 February, less than five days prior to the BMA’s announcement. The planned strike action is expected to lead to thousands of operations being cancelled.

Mr Hunt announced his decision to impose a contract earlier this month, following the collapse of negotiations earlier this year. The new contract is due to be phased in from August.

The BMA’s legal action is focused on the government’s alleged failure to carry out an equality impact assessment of the contract under the Equalities Act 2010.

The lawyers’ guidance makes clear that “there is nothing inherently unlawful about the proposed contract” adding: “Judicial reviews are always difficult claims to bring, and success is far from guaranteed, however Counsel advises that at this stage it appears the claim has greater than 50% prospects of success.”

But the lawyers warn that the action has potential to be “much more expensive” than typical judicial reviews adding: “Furthermore there are features to it that are likely to pose a considerable drain of the BMA’s internal resources.”

They continue: “Whether it is worth the investment will in large part depend upon how the [junior doctors’ committee] decides to deploy any leverage the case can generate to the benefit of members.”

The lawyers accept there is nothing to prevent the health secretary reconsidering equality issues and making the same decision to impose a contract again.

The memo says that while there are inequalities in the new contract they are “indirect discrimination”, which the lawyers say is “lawful when it is seen to be a proportionate means of achieving a legitimate aim”.

It adds: “[It] is not anticipated that there would be any unlawful discrimination in a decision to impose the proposed new contract.”

The lawyers say bringing the judicial review could cause a delay to imposition which could last until after the 2016 August rotation for junior doctors, although this is far from guaranteed. They add: “[Perhaps] most importantly, the [judicial review] could present the [junior doctors’ committee] with a last window of opportunity to salvage something from the negotiations and improve the contract that is about to be imposed.”

They note: “If we were the lawyers acting for the [secretary of state] we would advise him to respond to the JR by simply undertaking an [equality impact assessment] now and making a fresh decision to impose – at which point the potential illegality of the first decision would be irrelevant.

“The BMA could in theory still proceed to Court to seek a declaration as to the illegality of the first decision although it is questionable whether doing this would be worth doing in view of the time and expense involved.”

It is understood that the costs of judicial review proceedings could run into hundreds of thousands of pounds on both sides. News of the legal advice is likely to raise serious questions about why the BMA chose to make its planned legal action public so rapidly, given that its lawyers suggested keeping it secret for one to two weeks might present the best opportunity to reopen negotiations and extract improvements to the contract for the benefit of its members.

There will also be questions about the validity of pursuing legal action that will be a drain on taxpayers’ resources, and a “considerable drain” on the finances of the union itself, given its limited chances of preventing the contract imposition.

Questions will be asked as to whether an opportunity was missed to de-escalate an industrial dispute that threatens to drag on for a long time, with very serious implications for patients and the health service.