British Prime Minister David Cameron has said keeping the United Kingdom together is of paramount importance.
Mr Cameron made the comments during Prime Minister's Questions in the House of Commons.
He was responding to concerns that its constituent nations could seek independence after the UK voted to leave the European Union in a referendum on 23 June.
"Keeping the United Kingdom together is an absolute paramount national interest for our country," he told parliament.
He also warned of "choppy waters" ahead for the UK economy following last week's vote to leave the EU.
Mr Cameron told MPs in the House of Commons that the warnings of market instability and damage to the UK made by the Remain camp during the referendum campaign were now coming to pass.
While Britain was in a strong position to deal with economic difficulties, it would be wrong to "belittle" the scale of the challenges ahead, he said.
"There's no doubt in my mind these are going to be difficult economic times," Mr Cameron told parliament.
"If we do see economic difficulties, one of the ways we have to react to that is to make sure that our public finances and economy remain strong ... so I don't think it would be right to suspend the fiscal rules," he said, rejecting a call from Labour Party leader Jeremy Corbyn for more investment.
Mr Cameron was answering questions in the House of Commons as leaders of the 27 other EU countries continued their summit in Brussels, in a highly symbolic portent of things to come.
After bidding an emotional farewell to EU summitry after his last trip to the Belgian capital as prime minister yesterday evening, Mr Cameron was not invited to join the second day of the meeting, where other government heads discussed how they will deal with the fallout from last week's Brexit vote in the UK.
Mr Cameron also delivered a scathing condemnation of Labour Party leader Jeremy Corbyn's half-hearted campaigning in the referendum as he told the Labour leader he should "reflect on" his own part in the failure of the Remain campaign to persuade voters to stay.
Mr Cameron told MPs that the mood among EU leaders gathering in Brussels yesterday for their first summit since the UK referendum was "one of sadness and regret".
But he said there was also an agreement that "the decision of the British people should be respected".
And he said he had agreed with fellow leaders that "we are not turning our backs on Europe and they are not turning their backs on us".
Mr Cameron said he told the meeting that a key factor behind the Leave vote was "great concern about the movement of people and the challenges of controlling immigration".
And - in a pointed message apparently directed at Tory advocates of Brexit like Boris Johnson and Michael Gove - he added: "Many European partners were clear that it is impossible to have all the benefits of membership without some of the costs of membership. That's something the new prime minister and cabinet will have to think about very carefully."
Mr Cameron insisted that there was "no great clamour" among EU leaders for the UK immediately to kick off the two-year process of formal negotiations on its future relations with the bloc. Instead, he said, there was an acknowledgement that "we need to take some time to get this right".
European Central Bank president Mario Draghi told the meeting that private-sector forecasts for eurozone growth were being downgraded by 0.3% to 0.5% over the next three years, mainly because of the predicted slowdown in the UK economy, the PM told MPs.
Brexit backers booed in Commons
Brexit campaigners were booed and heckled in the Commons as angry Remain-backing MPs took out their frustrations following the historic vote to leave the EU.
UKIP MP Douglas Carswell was booed while pro-Brexit Tory Bernard Jenkin was accused of being "guilty by association" as he criticised UKIP leader Nigel Farage.
Pressure on Boris Johnson intensified as one Tory former minister referred to him as "Silvio Borisconi" and Labour MPs could be heard shouting "where's Boris?"
Mr Johnson has faced criticism for failing to set out what happens next despite being the figurehead for the Brexit campaign and favourite to succeed Mr Cameron as Tory leader and prime minister.
