Tony Blair says Britain's exit from the European Union will involve negotiations of "extraordinary complexity" and require "serious statesmanship" to carry them out.
Writing in the Daily Telegraph, the former prime minister said "our nation is in peril" after the leave result in the 23 June referendum.
With his own party "effectively disabled" due to infighting, Blair added that it was up to the Tories to conduct exit negotiations "with genuine patriotic regard for our nation's interest".
"There is going to be a negotiation of extraordinary complexity where there are a thousand devils in every detail," the ex-PM wrote.
"Those we used to call 'our European partners' are, unsurprisingly, divided and uncertain themselves. Some want us out fast. Some agree to delay the Article 50 process. This needs serious statesmanship."
There are fears that Brexit could trigger a chain reaction that causes other Eurosceptic countries to leave the EU, while there are also concerns that the UK itself could break up after both Scotland and Northern Ireland voted to stay in the single market.
Blair questioned Theresa May's plans to have a pro-Leave minister conduct negotiations with the EU if she becomes prime minister.
He was also critical of Ukip leader Nigel Farage's mocking address to the European Parliament earlier this week, where he told MEPs that they were "in denial" about Europe's problems.
"Remember who has to agree any new deal for Britain: the European Parliament," the Labour politician wrote.
He added: "Before any formal negotiation begins, we need to get a high level sense of where the boundaries are going to be, the things that might be compromised, the things that are red lines.
"The psychology of the other 27 countries is crucial to feel and shape: they could decide that other secessionist movements should be deterred and so be disinclined to flexibility; or they could decide that the British view – especially on immigration – reflects something strong across Europe and have a measured response which tries to accommodate that sentiment.
"Our nation is in peril. To allow us to come safely through this we need to be adult in our politics, to proceed with calm, maturity and without bitterness; because our future as a nation in the world and as the UK itself is at stake."