'Brussels will demand another £13.5billion in Brexit divorce payments if Theresa May seeks an extension to Article 50, it is claimed.
EU states are said to be 'hardening' their stance against a longer Brexit process and could force Britain to stay in a customs union as the price of agreeing a delay.
MPs could vote on a postponement this week if, as expected, Theresa May's deal is defeated again in the House of Commons tomorrow.
The PM has suggested a three-month delay but EU diplomats will demand more money if the extension is longer than a few weeks, the Daily Telegraph reported.
An EU source told the newspaper: 'Lines are hardening against extension... Anything more than a few weeks will come with legal and financial conditions attached.'
Brussels could ask Britain to pay an extra £13.5billion a year - £1bn a month - on top of the current £39bn divorce deal, it is reported.
The EU could also push for Britain to stay in a customs union, a move opposed by many Brexiteers who want the UK to strike new trade deals instead.
All 27 of the EU's remaining member states would have to agree an extension beyond March 29 if it is requested by Mrs May.
A plane was reported to be on standby at RAF Northolt to fly Mrs May to Brussels to clinch an agreement if there was any sign of a deal emerging from talks over the weekend.
But there has been no sign of a breakthrough and Mrs May will have to take her deal before Parliament on Tuesday with the controversial backstop still in place.'
Read more: EU 'will charge Britain £1billion a month' over any Brexit delay if Theresa May is forced to ask for an extension this week
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