(BBC) -- The UK is to offer Gibraltar continued barrier-free access to finance markets after Brexit, the BBC understands.
The UK is negotiating for Gibraltar to be treated the same as Britain when it leaves the EU in March 2019.
But the EU insists Madrid can stop a transitional deal or future trade relationship applying to Gibraltar unless there is a Spain-UK agreement.
Gibraltar fears Spain could use this veto to force talks about the Rock's constitutional future.
At the very least, it fears there could be talks about closer cooperation with Spain.
UK has 'moral' duty to Gibraltar on Brexit
If Britain refused, then Gibraltar could fall out of the EU next March without a transitional deal and lose its access to the British financial markets.
Ministers are to give the commitment to financial market access at a meeting in London of the so-called Joint Ministerial Council with the Gibraltar government.
Gibraltar is hoping to get some formal reassurance that it will retain access come what may.
This matters not just because most of its financial trade is with the UK but also because its insurance sector is important to Britain.
The Gibraltar government says that one in five British drivers insure their cars with firms based in Gibraltar. For insurers to offer annual policies with certainty, they needed some kind of reassurance before the end of this month.
The Lib Dem leader, Sir Vince Cable, told the BBC Theresa May should make rejecting Spain's veto a red line in her negotiations.
The prime minister, he said, had ignored the people of Gibraltar and taken her eye off of a key national interest.
He said: "If the government is going to take a tough line on Brexit in these negotiations, this is one of the things they should be tough about.
"Currently they have been very, very weak and created an enormous sense of anxiety and insecurity".
He added: "It is an issue of fundamental principle. Gibraltar has been attached to the UK for two centuries.
"We have seen off repeated demands by Spain to have control over the Rock. We should not allow Brexit to be used as a cloak for giving away what is a substantial British commitment."
Mr Cable said he wants the government to take an "absolutely firm stand" and remove "any question of Spain having a veto over Gibraltar's sovereignty" in its next stage of negotiations.
A UK government spokesperson said: "The EU's guidelines are a matter for the EU and the other member states.
"The prime minister has said that as we negotiate these matters we will be negotiating to ensure that the relationships are there for Gibraltar as well.
"We are not going to exclude Gibraltar from our negotiations for either the implementation period or the future agreement."