There was yet another thud on the doormat of Number 10, Downing Street earlier this week.

The European Research Group had gotten their heads together again and penned another little advisory note to the prime minister.

This isn't the first time the backbench group of eurosceptic MPs have shoved Brexit red lines through Theresa May's letter box of course.

This time was different though. It was more menacing. Although ERG chairman Jacob Rees-Mogg is always unfailingly polite when talking about the PM, the tone had suddenly changed.

The minister for the 18th Century claimed his 'report' for Mrs May which highlighted numerous reasons why a customs partnership would not work was not an 'ultimatum'. Yes it was Mr Rees-Mogg.

In normal circumstances these cliques of MPs can be ignored. But politics in 2018 is anything but normal.

The ERG have the numbers to spark a leadership challenge for Mrs May. Mr Rees-Mogg might appear harmless enough when he is interviewed on television but he has very serious ambitions to climb to the very top of the Conservative Party – Brexit is an excellent opportunity to do that.

Another aspect that set this ERG intervention apart from the previous letters was some cabinet ministers had been shown the report before the PM was even aware it existed.

This was targeted and timed to perfection to hit the press just prior to the latest Brexit war cabinet meeting. Ahead of the local elections and with Amber Rudd falling on her sword only days before, the prime minister didn't need the trouble.

So, facing infuriating the Brexiteers, Mrs May put off any decision on the customs partnership. But she can't put it off forever.

Sooner or later the prime minister needs to either take on the Remainers in her ranks – now including freshly ditched former home secretary Ms Rudd – or the Brexiteers.

By promising the UK will leave the customs union and that there will be no return to a hard border in Ireland the prime minister has ensured she will have to square up to one wing of her party on at some stage.

And now the local elections are in the books the time may well have come.

The majority of the 317 Tory MPs in the House of Commons backed remaining in the EU in 2016. If there was a free vote on forming some kind of customs partnership it is likely most Tories would back the plan.

Around 60 Tory MPs are believed to have been behind the ERG report. Within her own party Mrs May probably has the numbers to lay her own leadership on the line, prompt a challenge from the Brexit wing and win.

She why doesn't she spike their guns and come out swinging? Is it time for Mrs May's 'put up or shut up' moment?

On June 22, 1995, embattled Tory prime minster John Major strode to a lectern in Number 10's rose garden and spoke to the assembled press: 'For the last three years I've been opposed by a small minority in our party. During those three years there have been repeated threats of a leadership election ...

'It undermines the government and it damages the Conservative Party. I am not prepared to see this party I care for laid out on the rack like this any longer. To remove this uncertainty I have this afternoon tendered my resignation as leader of the Conservative Party.'

He closed by saying: 'Should I be defeated, which I do not expect, I shall resign as prime minister and offer my successor my full support.

'The Conservative Party must makes its choice. Every leader is leader only with the support of his party. That is true of me as well. That is why I am no longer prepared to tolerate the present situation. In short, it is time to put up or shut up.'

The issue that was tearing the party apart back in 1995 was Europe.

He won.

As it stands Mrs May is not only locked in laborious negotiations with Brussels but punishing infighting at home.

She lost a lot of her authority along with her majority which was always going to make the process of leaving the EU doubly tricky. But she needs to find a way to reassert her power, she needs to prove she is brave and can lead.

Mrs May must make a choice on the customs partnership. She needs to stop this head-in-the-sand politics and demand those who are not backing her 'put up or shut up'.