* EU member states agree 3-month flexible extension
* PM will respond to EU extension offer after reviewing it
* France accepts delay after UK election prospects rise
* Parliament to vote Monday on Johnson election demand (Adds U.S. bond yields, time of vote in parliament)
LONDON/BRUSSELS, Oct 28 (Reuters) - The European Union agreed a three-month flexible delay on Monday to Britain's exit from the bloc as Prime Minister Boris Johnson pushes for an election after opponents forced him to request an extension he had vowed never to ask for.
Just days before the United Kingdom is formally due to leave the EU on Oct. 31 at 2300 GMT, Brexit is hanging in the balance, with British politicians no closer to reaching a consensus on how, when or even if the divorce should take place at all.
Johnson, who won the top job in July by vowing, "do or die", to deliver Brexit on Oct. 31, was driven to request a postponement after he was defeated in parliament over the sequencing of the ratification of his divorce deal.
The 27 countries that will remain in the EU agreed on Monday to put off Brexit until the end of January, with an earlier departure possible should the faction-ridden UK parliament ratify the separation deal Johnson agreed with the bloc.
"The EU27 has agreed that it will accept the UK's request for a Brexit 'flextension' until 31 January 2020," European Council President Donald Tusk said in a tweet, referring to the concept of a flexible extension.
EU member states will need Britain to formally reply to its offer of a three-month delay before launching a "written procedure" whereby governments will have 24 hours to accept or reject.
"We can only launch the written procedure when we have the agreement of the UK government on the text," a senior EU official said. Johnson, who lacks a majority in parliament, is obliged by legislation passed last month to accept an extension once offered.
Johnson will respond to the EU's delay offer once he has reviewed the details, his spokesman said on Monday.
U.S. government bond yields rose on the news. The pound was trading flat at $1.2853.
With British politics still paralysed over carrying out Brexit 3-1/2 years after a 52%-48% referendum vote in favour of leaving the EU, Johnson is demanding parliament approve an election on Dec. 12 in return for more time to adopt his deal.
But he needs the support of two-thirds, or 434, of the 650 lawmakers for a new election. A House of Commons vote was due around 1900 GMT. The main opposition Labour Party was expected to abstain in the vote.
Britain's departure has already been delayed twice - from March 29 and April 12 - after Johnson's predecessor, Theresa May, failed three times to get her deal ratified by parliament.
The EU, forged from the ruins of World War Two as a way to prevent another ruinous conflict in Europe, is fatigued by Britain's protracted Brexit crisis but keen not to be held responsible for an economically tumultuous "no-deal" rupture.
French President Emmanuel Macron had been the main hurdle to an extension, arguing there had to be a good reason for a delay and that the British needed to break their own political deadlock. But a source close to Macron said the prospect of an election in Britain had strengthened significantly.
The source stressed that the third Brexit delay would come with conditions, including a refusal to renegotiate the divorce agreement and giving a green light to other EU countries to meet without Britain to discuss the bloc's future.
An EU diplomat said the hope was that the British would use the postponement wisely. Even though the United Kingdom is set to leave, it will have a legal obligation to nominate a new commissioner for the EU's powerful executive body.
The latest delay plan envisages that Britain could be out on Dec. 1 or Jan. 1 should parliament ratify the agreement in November or December respectively, according to diplomats who deal with Brexit in Brussels.
But in London, there was no consensus on an election which Johnson says is needed to overcome the impasse.
Pressure on Labour was increased by two other parties, the Liberal Democrats and the Scottish National Party (SNP), which called for a Dec. 9 election on the condition that Johnson give up his attempt to push his deal through parliament.
Downing Street sources indicated that if their attempt on Monday to force an election failed, then the government could introduce a bill similar to the one proposed by the Liberal Democrats and SNP - a vote that would require only a simple majority in the 650-seat parliament.
(Additional reporting by Elizabeth Piper and Kylie MacLellan; Writing by Guy Faulconbridge Editing by Mark Heinrich and Catherine Evans)