* May, Davis to dine with Juncker, Barnier at 1630 GMT
* Meeting long planned before summit, London says
* Talks did not feature in Juncker, Barnier schedules
* EU leaders to meet May on Thursday in Brussels (Adds Juncker spokesman, diplomats)
LONDON/BRUSSELS, Oct 16 (Reuters) - Prime Minister Theresa May is unlikely to offer new concessions to Brussels over the terms of Britain's divorce from the European Union when she meets European Commission chief Jean-Claude Juncker over dinner on Monday, her spokesman indicated.
After deadlock in Brexit talks appeared to dash May's hopes that a summit later this week could launch negotiations on future trade ties, the spokesman said May was hoping to move matters forward "in a constructive manner" on Monday night.
Other EU governments insist they will only open trade talks if May tells them if and how she plans to pay an exit bill they put at tens of billions of euros.
Asked if May would flesh out details of what Britain is prepared to offer in terms of a financial settlement, the spokesman said: "The PM set out her position in the Florence speech in relation to the settlement, and that's where we are."
The 90-minute meeting, due to start at 6:30 p.m. (1630 GMT),
will also be attended by the bloc's Brexit negotiator Michel Barnier and his British counterpart, David Davis.
"It will either be a very expensive dinner, costing about 30 billion euros or else -- what is this?" one EU diplomat said.
After talks with Davis last week, Barnier said negotiations were deadlocked, notably over London's refusal to detail what it was offering to pay Brussels. This followed an attempt by May last month to revive the negotiations with a speech in Florence promising Britain would honour its EU commitments.
As a result, Barnier told European Union leaders not to launch the talks on a future relationship that May has demanded. As time ticks down to withdrawal in March 2019, concern is rising across Europe that the process may collapse.
"This is about reflecting upon Florence and the constructive way it's been received and that we want to continue to move forward in a constructive manner," May's spokesman said.
British Foreign Secretary Boris Johnson also urged progress in the talks: "Let's get these negotiations going and stop letting the grass grow under our feet."
But EU leaders say May has been too vague in her offer of a financial settlement -- something many diplomats believe is due to a fear that to agree even a very rough a figure would spark a backlash from hardline Brexit supporters such as Johnson.
British officials say they cannot give a figure on the final bill until they know what kind of trade agreement will be sealed during the talks.
In response to suggestions from Barnier, EU governments have agreed, however, that the summit on Thursday and Friday should give him a green light to explore a possible post-Brexit transition period - although only in internal discussions within the EU, not with the British negotiators themselves.
Even that has run into resistance, notably from heavyweights Germany and France. They insist further progress in the British divorce package is required for any such gesture to be made to May, who is struggling to unite her own government behind her plan to reach a deal on a two-year transition during which Britain could stay in the single market and customs union.
A statement by the 27 other EU states, planned for Friday when their leaders will meet after May has left, is being redrafted to harden the conditions under which Barnier will be allowed to explore the options for the transition.
Without meeting three key tests, the Union says there can be no opening of talks on what happens after March 29, 2019. The leaders have been expected to say they hope that they can launch that second phase of talks after their next summit in December.
Business leaders on both sides of the English Channel have said that without clarity by the new year, they will increasingly have to take investment decisions to reflect uncertainty over British access to EU markets.
After a similar meeting at 10 Downing Street in late April, leaked accounts of Juncker's "shock" at what he said were unrealistic British expectations irritated May.
Asked about his expectations for Monday's meeting, Juncker told reporters: "We will talk, and you will see the autopsy." (Additional reporting by Gabriela Baczynska and Robin Emmott in Luxembourg; Writing by Alastair Macdonald; Editing by Jeremy Gaunt)