Theresa May is facing growing anger over her secrecy about Brexit, after Boris Johnson appeared to leak a key aspect of the plan on a foreign trip.

Both Jeremy Corbyn and the SNP let rip at the Prime Minister after the Foreign Secretary told a Czech newspaper that Britain will “probably” leave the EU’s customs union.

Critics say the move would pile damaging extra costs and red tape on to businesses, which would lose the right to sell into the single market without form-filling and customs checks.

Leaving the customs union would also require large numbers of extra civil servants and border staff to police entry points, an inquiry by MPs was told today.

But Downing Street has refused to confirm Mr Johnson’s comments – insisting “a decision has not been taken yet”.

At Prime Minister’s Questions, that stance was fiercely criticised by both the Labour leader and Angus Robertson, the leader of the Scottish nationalists.

With Mr Johnson sitting just a few places away from Ms May on the Commons front bench, Mr Corbyn called on him to step forward and come clean.

“He’s only a few places down from her,” he said, adding: “Surely we would all be better informed if he did?”

Mr Corbyn said: “I sympathise with the Italian government minister this week, who said about her Government, ‘Somebody needs to tell us something, it needs to be something that makes sense’”

And he added: “We have a Prime Minister who is not prepared to answer questions on what the Brexit strategy actually is.”

The attack was echoed by Mr Robertson, who said: “On the day that we hear that ‘post-truth’ has become the international word of the year, we have a running commentary from the Foreign Secretary.

"He is prepared to tell the media in the Czech Republic that the United Kingdom is likely to leave the EU customs union post-Brexit, but still wants to trade freely afterwards.

“Will the Prime Minister confirm today to Parliament, to the country, whether the UK is likely to leave the EU customs union post-Brexit, yes or no?”

But Ms May replied that membership of the customs union was “not just a binary decision” – apparently reflecting the possibility that some business sectors could be in it, while others leave.

Earlier, she ducked the issue all together in answer to Mr Corbyn, saying: “We are preparing carefully for the formal negotiations. What we want to ensure is that we have the best possible trading deal with the European Union one we have left.”

The Prime Minister added that revealing Britain’s hand to the EU “would be the best possible way of ensuring we got the worst possible deal for this country – and that’s why we won’t do it”.

On Tuesday, the Foreign Secretary told the Czech newspaper: “Probably, we will need to leave the customs union, but this is a question which will be dealt with in the negotiations.”