Theresa May should make clear whether she aims to keep Britain in the European single market and customs union when she publishes her Brexit negotiations plan, a cross-party committee has said.
And the House of Commons Exiting the EU Committee said that the Prime Minister must also commit to giving MPs a vote on the final deal which she secures with the remaining 27 members.
The committee said that Mrs May must publish her plan in the form of a White Paper by mid-February, to give MPs a chance to debate it before the Prime Minister kicks off negotiations by tabling Article 50 in March.
The Prime Minister signed up to a Labour motion requiring her to publish details of her plans for Brexit before the invocation of Article 50, but has so far given little indication of what it will contain. The PM - who will deliver a major speech on Britain's post-EU future on Tuesday - has also held back from revealing whether MPs will be granted a vote on the Brexit deal.
In its first report, the committee - which includes prominent Brexiteers like Michael Gove, John Whittingdale and Dominic Raab, as well as former backers of the Remain campaign - warned that the task of preparing for Brexit was placing "strain" on departments across government.
Managing and implementing the decisions required to prepare for life outside the EU will be a "significant challenge" for the civil service for a matter of years and "may well" require the recruitment of more staff, it warned.
The report said ministers should seek an outline framework of the UK's future trading relationship with the EU as part of the Article 50 negotiations, with appropriate transitional arrangements to allow trade to continue normally if a deal is not reached in time for the expected Brexit date of 2019. But it warned that this "will not be in the Government's gift to deliver", as the EU may insist on delaying trade talks until after the withdrawal negotiations are over.
It would be "unsatisfactory and potentially damaging" to both the EU and UK if Britain tumbled out of the bloc without a deal when the two-year Article 50 deadline expires, the committee warned. An abrupt "cliff edge" departure would be "extremely disruptive" and the absence of transitional arrangements could push some businesses to relocate out of the UK or move investment overseas.
The committee argued that the Government should seek to ensure continued access to EU markets for UK financial services providers - whether through the extension of "passporting" rights or an agreement on mutual recognition of regulations. And it said it was "essential" that co-operation should continue on defence, foreign policy, security, financial crime and counter-terrorism.
Committee chair Hilary Benn said: "This is going to be a hugely complex task and the outcome will affect us all.
"Whatever deal is concluded, Parliament must be given a vote on it and the Government should make this clear now."
The report also called for devolved administrations in Scotland, Wales and Northern Ireland, as well as the English regions, to be "duly involved" in the Brexit process and have their views taken into account. It was "essential" that the stability of Northern Ireland is not jeopardised by EU withdrawal.
Conservative committee member Karl McCartney said the report was "flawed" in its call for transitional arrangements on trade and tariffs, which he said would "merely prolong the uncertainty for businesses and everyone else".
He said he did not accept "emotive and negative language from the Remain side" which had found its way into the report.
Committee members from other parties should have taken a more "collegiate" approach, said the Brexit-backing Lincoln MP, adding: "These MPs, I fear, are seeking to thwart the democratic will of the 17.5 million people who voted to leave the European Union by taking part in the largest democratic vote our country has had."
A Department for Exiting Europe spokesman welcomed the report, adding: "As the report acknowledges, the negotiations will not be easy but we approach them with the full intention of securing a deal that delivers the best possible outcome for the UK.
"We want a smooth and orderly exit from the EU, and will look at the best way to deliver that."
Joe Carberry, co-executive director of the Open Britain campaign, said the "vital" report had unified Leave and Remain politicians around demands for greater transparency from the Government.
"Parliament should not be sidelined during the Brexit negotiations," said Mr Carberry. "That means before they start, the Government should bring a White Paper before Parliament with their substantive plans and, at the end of the process, it is imperative that MPs are given a vote on the final deal."
John Longworth, co-chairman of the Leave Means Leave campaign, said it was "absurd" to assume that transitional arrangements would be required, as two years would be "more than enough time" to seal a trade deal.
"Provided the Prime Minister makes clear now the direction of travel, that we are leaving the single market and the customs union, business will have plenty of time to plan," he said.
"The Government must enter the negotiations with the aim of getting the best deal for Britain. If this isn't possible the Government must waste no time and be prepared to walk away - with or without a trade deal."