THERESA MAY has endured some difficult moments during her tenure as Prime Minister, but the next few days are likely to prove as challenging as anything she has experienced.

When Jeremy Corbyn confirms Labour’s desire to remain in a post-Brexit customs union in a speech later today, he will put clear water between his own party’s position and that adopted by the Conservatives.

Thus far, Mrs May has made a pretty decent job of holding the competing factions of the Tory party together, but her task has been aided by the lack of a viable alternative for ‘Remainers’ to rally around.

Now, that vacuum no longer exists, and as MPs prepare to vote on the Government’s policy of staying out of the customs union, there is the real risk of pro-Remain MPs such as Ken Clarke and Anna Soubry voting with Labour.

It will not take many defectors to overturn the Government’s wafer-thin majority, but if Mrs May backtracks in an attempt to win the bill, she will almost certainly turn the far right of her party against her.

In winning the battle against the likes of Mr Clarke and Mrs Soubry, she risks losing the war against the likes of Boris Johnson and Jacob Rees-Mogg.

In her negotiations so far, Mrs May has managed to appease both sides, but the uneasy truce will not last forever.

Mr Corbyn’s speech later today will turn the focus squarely on the Prime Minister. Her response will almost certainly determine her political trajectory for the final 12 months of Brexit negotiations.