LONDON, March 19 (Xinhua) -- British and European Union negotiators announced Monday their biggest leap forward in Brexit negotiations. But the optimism shared by some was not welcomed by critics of the breakthrough announced in Brussels.

The biggest stumbling block remains the question of the border between the Irish Republic and Ireland when Britain leaves the European Union (EU).

The two sides announced an agreement has been reached on the relationship between both sides until the end of 2020, almost two years after Britain ends its membership next March.

The Confederation of British Business (CBI), which represents the country's biggest companies, said the breakthrough had brought a welcome gift for firms on both sides and had helped lift a cloud of uncertainty for businesses.

CBI director-general Carolyn Fairbairn, said: "Agreeing transition is a critical milestone that will provide many hundreds of businesses with the confidence to put their contingency planning on hold and keep investing in the UK.

"This is what businesses have been calling for since last summer. This is a victory for common sense that will help protect living standards, jobs and growth. It shows what can be achieved when people and prosperity are placed above politics and ideology," Fairbairn said.

He said other hurdles on the Brexit path now need to be cleared in the same spirit, including urgent resolution of the Irish border.

Adam Marshall, director general of the British Chambers of Commerce, also welcomed the deal, saying it was the milestone many businesses across Britain have been waiting for.

"The agreement of a status quo transition period is great news for trading firms on both sides of the channel, as it means that they will face little or no change in day-to-day business in the short term.

"While some companies would have liked to see copper-bottomed legal guarantees around the transition, the political agreement reached in Brussels is sufficient for most businesses to plan ahead with a greater degree of confidence," Marshall said.

Keir Starmer, from the main opposition Labor party who is shadow Brexit secretary, accused ministers in Prime Minister Theresa May's government of wasting time by fighting among themselves and pursuing their reckless red lines.

"It is welcome that they have finally struck a deal on transition and now the government must prioritize negotiating a final agreement that protects jobs, the economy and guarantees there will be no hard border in Northern Ireland," said Starmer.

Nigel Farage, co-founder and former leader of the anti-EU party UKIP, accused the prime minister of a climb down and said May should resign, saying: "she is totally useless and must go."

A spokesperson for the Leave Means Leave campaign accused the government of caving in to the EU, adding: "Under the current plans, we will not be taking back control of our fisheries when we leave the EU, something which the government had previously promised.

Meanwhile, Open Britain issued a statement from Labor MP Chuka Umunna, in which he said Monday's deal shows that Brexiters have had to surrender on almost every single point.

"Despite once claiming they held all the cards in the negotiations, in the end the Brexiters have been prepared to compromise and surrender on almost every single point," said Umunna. It would have made more sense for Britain to extend the Article 50 period and remain for a while longer in the EU beyond next March, Umunna added.

Although the EU and Britain have provisionally agreed how the transition period will operate, they have yet to finally settle how to handle the Northern Ireland border issue.

There are fears a wrong solution could threatened the so-called Belfast Agreement which ended decades of conflict in Northern Ireland.

Former British diplomat Jonathan Powell, who worked on the negotiations that led to the return of Hong Kong to China, was also involved in the Belfast peace agreement.

Writing Monday night in the London-based Independent, Powell warned that Theresa May's failure to deal with problems posed by Northern Ireland's border threatens to bring Brexit negotiations "crashing down".

Concern was also expressed in Scotland where the fishing industry reacted angrily to the Brexit deal which will see Britain consulted on fish quotas and access to its waters until 2021.

Bertie Armstrong from the Scottish Fishermen's Federation: "We will leave the EU and leave the Common Fisheries Policy, but hand back sovereignty over our seas a few seconds later."

Britain's Brexit Secretary David Davis said the agreement is conditional on both sides agreeing a final withdrawal treaty, adding today's development would smooth the path to a future permanent relationship.

The focus will now shift to the summit meeting later this week of European leaders who will be asked to sign-off the transitional period deal announced Monday by Davis and the EU's chief negotiator Michel Barnier.