* UK-EU talks are taking serious turn
* EU summit on Oct. 17-18
* PM Johnson to update British ministers
By William James
LONDON, Oct 13 (Reuters) - Brexit talks with the European Union aimed at striking a last-minute divorce deal are getting serious, Britain said on Sunday, as Prime Minister Boris Johnson prepared to update his cabinet on the state of negotiations.
In a pivotal week that could decide the future of Brexit and the fate of the world's fifth largest economy, Johnson is trying to strike an exit deal with the EU to allow an orderly departure on Oct. 31.
But Johnson must navigate the complexities of EU politics to strike a deal at an Oct. 17-18 EU summit and then try to convince a deeply divided British parliament on Oct. 19 to ratify any deal.
If he succeeds, Britain will leave the bloc on Oct. 31 with arrangements to minimise disruption at borders and preserve the complex supply chains that underpin swathes of the economy.
If he fails, Britain faces an uncertain future - lawmakers will begin a battle to delay Brexit that could end up being decided in the courts, while Johnson will fight for a no-deal exit at the end of the month to ensure his political survival.
British and EU negotiators are working through the weekend see if they can turn proposals put forward by Johnson, which were initially rejected by the EU, into a deal that works for both sides.
"Negotiations seem to be taking a more serious turn and that's encouraging," Jacob Rees-Mogg, a Brexit-supporting Conservative lawmaker who runs the government's business in parliament, told Sky News.
"It is always difficult to put specific odds on things but it's certainly looking more positive this week than it did last week," he said.
Johnson is due to brief cabinet members on progress in those talks by conference call at around 1145 GMT.
If Britain leaves the EU without a deal, many expect serious short-term disruption with possible food, fuel and medicine shortages, and long term damage to Britain's reputation as a safe and stable home for foreign investment.
The government says it is doing everything it can to get a deal, and that it has contingency plans to mitigate the impact of a no-deal exit.
Ireland has proved the toughest issue in the Brexit talks: specifically how to prevent the British province of Northern Ireland becoming a backdoor into the EU's markets without having border controls.
Ireland fears controls on the 500-km (300-mile) border with Northern Ireland would undermine the 1998 Good Friday Agreement which ended three decades of sectarian and political conflict that killed more than 3,600 people.
A WEEK IS A LONG TIME
To leave with a deal on Oct. 31, Johnson has just days to convince the EU to accept a compromise on border arrangements and then get leaders to sign off on a deal at an Oct. 17/18 EU summit in Brussels, likely to last late into the night.
Although the talks have made more progress than many expected a week ago, sources in Johnson's office have said that a deal remains a long way off, echoing caution on the EU side of the negotiations.
The Sunday Times reported that Johnson will speak to German Chancellor Angela Merkel, French President Emmanuel Macron and European Commission President Jean-Claude Juncker by the end of Monday.
If successful in Brussels, Johnson then has to win a vote in the divided parliament that rejected his predecessor Theresa May's deal three times earlier this year. A vote is expected to take place in a rare Saturday session of parliament on Oct. 19.
Since becoming prime minister in July, Johnson has yet to win a vote in parliament and, after kicking 21 lawmakers out of his party for not backing his Brexit plans, is well short of a majority.
While his opening bid in the latest phase of negotiations with the EU received cautious endorsement at home that suggested it could find the necessary support in parliament, much will depend on the final shape of any deal.
The three main opposition parties, Labour, the Scottish National Party and Liberal Democrats are expected to oppose it.
That leaves Johnson's hopes pinned to a group of Labour rebels and the small party which notionally keeps him in power, Northern Ireland's Democratic Unionist Party.
The DUP position is unclear, after the party's deputy leader Nigel Dodds said the mooted eleventh-hour solution to the deadlock over Brexit could not work. (Reporting by William James; editing by Guy Faulconbridge)