Julian Smith has lashed out at Theresa May’s ministers as he criticised them over what he called the worst example of cabinet discipline in British political history.

In an unprecedented intervention, the chief whip also criticised the government’s approach to Brexit, suggesting a softer exit from the bloc was inevitable after Ms May gambled away the Tories’ majority in 2017.

The remarks from Mr Smith, who also confessed he is “knackered”, came as MPs prepare for string of key votes in the Commons on different Brexit options during the second round of indicative votes later today.

“The thing that people forget is that the Conservative Party went to get a majority in order to deliver Brexit [and] failed to get a majority,” the chief whip told the BBC.

He added: “The government as a whole probably should just have been clearer on the consequences of that. The parliamentary arithmetic would mean that this would be inevitably a softer type of Brexit.”

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While the strategy was apparently misjudged, Mr Smith said he was “frustrated” by MPs who “don’t see the light as clearly as I do”.

However Mr Smith highlighted that a lack of discipline extended all the way to the cabinet, with ministers “sitting around the cabinet table ... trying to destabilise her [Ms May]”.

Pointing to the lack of discipline at the top levels of government, he claimed: “As you’re aware, discipline is not as good as it should be. This is, I think, the worst example of ill-discipline in cabinet in British political history.”

Commons speaker John Bercow will later select the options on the order paper for the second round of indicative votes, but they could include negotiating a customs union with the EU, a “confirmatory” second referendum, and rejecting a no-deal exit.

While none of the options last week received a majority vote in the Commons, the motion put forward by senior Conservative Ken Clarke on the customs union was just six short from passing the threshold.

Speaking on Sunday, the justice secretary David Gauke said the UK was facing a “national crisis”, as he warned the prime minister she could not ignore the will of parliament if MPs unite around a customs union.

“If parliament is voting overwhelmingly against leaving the European Union without a deal but is voting in favour of a softer Brexit, then I don’t think it’s sustainable to ignore parliament’s position and therefore leave without a deal,” he said.

In the wake of Mr Smith’s comments, chief secretary to the Treasury Liz Truss told BBC Radio 4’s Today programme: “It’s not clear to me that going softer is the way to command support.”