THE UK will be plunged into uncertainty and may never leave the European Union if MPs reject the Brexit deal next week, Theresa May said.
But Taoiseach Leo Varadkar said the British government had failed to offer any solutions on the Withdrawal Agreement.
In a warning ahead of Tuesday's Commons showdown on her Withdrawal Agreement, the prime minister acknowledged "no one knows" what will happen if her plan is defeated.
Mrs May said both the democratic and economic cases for backing her deal are "clear", and issued a plea to MPs: "Let's get it done."
Speaking in Dublin, however, Mr Varadkar said the deal reached in November, including the Irish backstop, was "already a compromise" delivered in response to British requests.
"We were and remain happy to apply the backstop only to Northern Ireland if they want to go back to that, it doesn't have to trap or keep all of Great Britain in the single customs territory at all or for a long period," he said.
The prime minister is urging Brussels to give ground in order to help her deal survive Tuesday's crunch vote by agreeing changes to the Northern Ireland backstop measures.
In a speech in Leave-voting Grimsby, Mrs May said: "Next week MPs in Westminster face a crucial choice: Whether to back the Brexit deal or to reject it.
"Back it and the UK will leave the European Union. Reject it and no one knows what will happen.
"We may not leave the EU for many months, we may leave without the protections that the deal provides. We may never leave at all.
"Everyone now wants to get it done, move beyond the arguments, past the bitterness of the debate and out of the EU as a united country ready to make a success of the future."
In a plea to Brussels for support in making changes, she said what the European Union does over the next few days "will have a big impact on the outcome of the vote".
Aiming her words directly at EU leaders, she said: "Now is the moment for us to act. We've worked hard together over two years on the deal. It's a comprehensive deal that provides for an orderly exit from the EU and sets the platform for an ambitious future relationship.
"It needs just one more push to address the final specific concerns of our Parliament. So let's not hold back. Let's do what is necessary for MPs to back the deal on Tuesday."
Mrs May also sent a message of warning to hardline Brexiteers considering voting against her deal next week.
Any delay to the Withdrawal Agreement could lead to "a form of Brexit that does not match up to what people voted for", or to a second referendum, she said.
A softer Brexit deal "could mean no end to free movement, no ability to strike our own trade deals, no end to the big annual payments, no taking back control – which is what the British people voted for", said Mrs May.
And she accused Labour leader Jeremy Corbyn of supporting "a divisive second referendum that would take the UK right back to square one".
"If MPs reject the deal, nothing is certain. We would be at a moment of crisis.
"MPs would immediately be faced with another choice. Either we leave the EU with no deal on March 29 – I don't believe that would be the best outcome for the UK or the EU – or we delay Brexit and carry on arguing about it, both amongst ourselves and with the EU. That is not in our interests either.
"More talking will not change the questions that need to be settled and delay risks creating new problems."
Mrs May warned that if the UK asks for a delay simply to give MPs more time to decide what to do, "the EU might insist on new conditions that were not in our interests before they agreed to such an extension".
Foreign Secretary Jeremy Hunt warned Brussels that a failure to co-operate on securing changes to the Brexit deal could "poison" relations with the EU for years.
"This is a moment of change in our relationship between the UK and the EU and history will judge both sides very badly if we get this wrong," Mr Hunt told the BBC Radio 4 Today programme.
"We want to remain the best of friends with the EU. That means getting this agreement through in a way that doesn't inject poison into our relations for many years to come."
The EU said technical discussions are ongoing and it insisted Brussels has come forward with ideas to resolve the deadlock.
European Commission spokesman Alex Winterstein told reporters in Brussels: "The EU side has offered ideas on how to give further reassurances regarding the backstop. You are aware of all this."
