LONDON, June 11 (Xinhua) -- Theresa May faces the toughest 48-hours of her premiership as she prepares a crucial "rallying the troops" meeting Monday with more than 300 Conservative Members of Parliament (MPs).

May is to address her party's MPs in a private meeting Monday night in the House of Commons ahead of a two-day debate starting Tuesday on her Brexit Withdrawal Bill.

The bill, which aims to convert all European Union (EU) laws into British law when the UK leaves the bloc in March next year, returns to lawmakers in the House of Commons after its stormy journey through the House of Lords where unelected members agreed on 15 amendments.

Britain's national newspapers were given a hint of the tone May will be using at the meeting of her backbench MPs, which is that backbenchers must come together to pass the government's Brexit legislation.

She will say: "The message we send to the country through our votes this week is important. We must be clear that we are united as a party in our determination to deliver on the decision made by the British people."

The tabloid Sun newspaper said Monday that May's rallying call will come ahead of a string of crunch votes that could define her premiership.

May will make her crucial address at the meeting of her MPs in a last ditch bid to stave off a damaging rebellion over her Brexit plans, the Sun added.

The newspaper reported that at least 15 pro-EU Conservative backbenchers have warned they may side with the main opposition Labor Party in four of the 15 House of Commons votes.

These include ensuring Britain stays in an EU customs union after Brexit and keeping Britain in the single market.

Former interior minister Amber Rudd, who recently resigned as Home Secretary, and former Conservative leader Iain Duncan Smith issued a joint statement Sunday urging Conservatives to back May in the crucial votes. They warned that not doing so risked bringing down her government with the prospect of paving the way for Jeremy Corbyn to take her seat as Labor prime minister.

Remain supporter Rudd and leading Brexiteer Duncan Smith put aside their differences to make their plea.

The Independent newspaper also said Monday that MPs were gearing up for a fraught two-day contest over the EU Withdrawal Bill, wherein the government is trying to overturn all but one of the 15 House of Lords amendments on issues ranging from customs to a meaningful vote for parliament on the final Brexit deal.

The newspaper said that with a very close result expected, Conservative Party managers will be ramping up the pressure on potential pro-EU rebels to ensure they have the numbers to win in this week's voting.

Former Labor prime minister Gordon Brown said May could be forced out if her government loses a crunch vote this week on its Brexit legislation.

Brown said one of the upcoming votes that would guarantee MPs a "meaningful vote" on a final Brexit deal with Brussels could be a deciding factor in May's future as prime minister.