Labour has accused the Government of planning significant cuts to the Border Force that would weaken UK security against a Brussels-style terror attack.
Shadow home secretary Andy Burnham renewed calls for its budget to be published after hearing from insiders that staff were told to anticipate 6% reductions this year and next - a total of £88 million.
He has written to Theresa May, warning it is the "worst possible time" to squeeze spending and criticising the "highly unusual" failure to release the budget just days before the start of the financial year.
The Home Secretary was "going to great lengths to hide these cuts" because she was aware of the dangers "in her heart of hearts", Mr Burnham said.
The Home Office said "all necessary measures" would be taken to protect the UK's border but conceded that the Border Force budget was still be be finalised.
A spokesman pointed to a £41 million investment in additional officers as part of a crackdown on border crime and millions spent improving security in French ports.
"Over the next four years, we will increase the counter-terrorism budget by £500 million and invest £130 million in state of the art technology at the border.
"All necessary funding and investment will be in place by the start of the next financial year."
Mr Burnham said: "This is not good enough. The public has a right to know about the Government's plans for the UK border and that is why I call on them to publish these figures without delay and to drop these damaging cuts.
"This decision is yet another consequence of George Osborne's botched Spending Review and Budget. It has nothing to do with the EU Referendum and everything to do with a Government with its priorities wrong and whose austerity drive is putting our country at risk."
In his letter he said he was "worried you are about to make a very serious mistake with respect to the resourcing of UK border security".
Mr Burnham wrote: "As you know, the Border Force has already experienced consecutive years of cuts in the last Parliament and is already stretched to the limit. It is in no position to absorb further cuts on this scale.
"So that there can be a proper debate about the Government's plans for the UK border, I would urge you to confirm without delay whether it is your intention to inflict cuts of this scale on the Border Force, and whether you think this can be achieved without compromising border security.
"It is my strong view that, in the wake of the terrible events in Brussels and Paris, this is the worst possible time to be cutting UK border security.
"As we learn more about the extent of the terror networks in Belgium, there will be increasing focus on the UK-Belgium border and whether security is strong enough across all air, sea and rail terminals.
"Now is the time to be strengthening our borders, not weakening them. I urge you to drop these damaging cuts."
The Home Office said Mrs May "will respond to Mr Burnham's letter in due course" adding that it was "not uncommon" for budgets to be finalised so close to the start of the financial year.