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David Cameron is to meet European Union leaders for the first time since the UK voted to leave.
The UK prime minister will discuss the implications of the Brexit vote and the way ahead at an EU summit in Brussels.
German, French and Italian leaders said on Monday there could be no "formal or informal" talks on a British exit at this stage.
Meanwhile, Health Secretary Jeremy Hunt has called for a second referendum over the terms of the UK's departure.
He said the withdrawal process should be delayed until shortly before the next election - meaning a delay until after 2022 at the earliest.
He is the first cabinet minister to go public with the call.
Mr Hunt, who is considering standing for the leadership after Mr Cameron stands down, said the UK must remain within the EU's single market.
He floated the idea of a "Norway plus" arrangement outside the EU, where the UK would enjoy the current trade benefits of being a full EU member while negotiating revised immigration rules.
The British public's concerns about immigration needed to be addressed, he said, and that was also in the EU's interest as it "faced collapse" unless the current right of all citizens to live and work in other member states was reconsidered.
In other developments on Tuesday, Chancellor George Osborne ruled himself out of replacing Mr Cameron as prime minister.
He said in the Times that he had fought hard for a vote for remaining in the EU, and though he accepted the referendum result "I am not the person to provide the unity my party needs at this time."
Labour MPs will vote on a motion of no confidence in their leader Jeremy Corbyn - who has insisted he is staying put - while Scottish First Minister Nicola Sturgeon will address MSPs over Brexit's implications for Scotland's future.
As Europe tries to come to terms with Britain's decision to leave, Mr Cameron will attend a working dinner of EU leaders after meetings with European Council President Donald Tusk and European Commission President Jean-Claude Juncker.
He will not attend talks between the leaders of the other 27 EU member states at breakfast on Wednesday.
'Swift and coherent'
Speaking on Monday, Mr Cameron said the UK must accept the referendum result and that a special unit within government was being set up to lay the initial groundwork for leaving the EU.
However, he has said that it must be up to his successor - who will be elected by the start of September - to decide how to proceed and precisely when to give formal notification of the UK's intention to leave by invoking Article 50 of the Lisbon Treaty.
Under EU rules, once this happens, the UK would have two years to negotiate the terms of its divorce from the EU - unless the remaining 27 members unanimously agree to extend the process. It must also negotiate its future trading relationship with the EU.
German Chancellor Angela Merkel has said a new UK government must be given time to prepare itself, but she also said the process cannot be delayed indefinitely amid fears of "contagion" for an organisation facing multiple economic and political challenges.
French President Francois Hollande and Prime Minister Matteo Renzi of Italy have agreed with her stance.
The European Parliament will also meet on Tuesday in an emergency session to debate the fallout from the Brexit vote - including a non-binding motion urging the "immediate activation" of Article 50.
MEPs, who must ratify any final agreement with the UK, have said they want to be "fully involved" in the process and that withdrawal must be "swift and coherent" in order to protect the interests of the wider "European project".
After two days of sharp falls in the stock market and sterling and political turmoil engulfing both the Conservative and Labour parties, there is increasing uncertainty about what Brexit will entail and the precise nature of the mandate that Mr Cameron's successor will be given.
Speaking in the Commons on Monday, former Chancellor Ken Clarke said it should be up to MPs to decide the terms of the UK's exit and the blueprint set out by the Leave campaign during the referendum - including quitting the single market - should not be sacrosanct.
But Commons leader and Leave campaigner Chris Grayling said the UK was the EU's biggest customer and, as such, negotiations would be a two-way process. There will be real damage to European markets if a "sensible agreement" were not reached, he said.