Post-Brexit vote uncertainty has intensified as the SNP threatened to veto withdrawal, Germany asked the UK to think again, and London told Brussels to back-off over demands for a speedy exit.

Warnings by Scottish First Minister Nicola Sturgeon that the Edinburgh parliament could block UK withdrawal by using devolved powers were met with scepticism by constitutional experts, but highlighted the unprecedented nature of the unfolding events.

And in a surprise intervention after days of turmoil triggered by the surprise EU referendum result, British politicians were urged by Berlin to reconsider the repercussions of leaving.

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German Chancellor Angela Merkel's chief of staff, Peter Altmaier, was reported as saying: "Politicians in London should have the possibility to reconsider the consequences of an exit."

The RND newspaper network quoted Mr Altmaier stating that a UK withdrawal would be "a difficult watershed with many consequences," before adding that Britain could rejoin at a later date "but that would take a long time".

The move came as Foreign Secretary Philip Hammond signalled Britain is set to resist pressure from Brussels and other EU capitals for a swift start to negotiations on its withdrawal from the bloc.

Despite a demand from European Commission president Jean-Claude Juncker - backed by the foreign ministers of France and Germany - for the process to begin "immediately", Mr Hammond insisted that "nothing is going to happen at the moment".

The timing of the formal notification of intention to leave the EU, set out in Article 50 of the Lisbon Treaty, is in Britain's hands and talks will not start until a new prime minister is chosen to replace David Cameron, he said.

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Mr Hammond told ITV1's Peston on Sunday: "Our European Union partners would like us to do it quickly and we're already under some pressure. But the Prime Minister is clear that the correct way to do this is for the new prime minister to make the decision. There is no imperative upon us to serve the notice at any particular time."

Foreign direct investment into the UK has already "all but dried up" as companies wait to see the consequences of the Brexit vote, the foreign secretary said.

Mr Hammond said that he and other Cabinet members - including Chancellor George Osborne - would remain in office until the selection of a new Conservative leader, expected by the time of the party's annual conference in October.

His comments came as Washington confirmed that Secretary of State John Kerry will come to London on Monday for talks with Mr Hammond as part of a frantic round of diplomacy as the US grapples with the fallout from Thursday's referendum which will also see him in Brussels.