The Treasury is very concerned about overspending on Northern Ireland's botched green energy scheme, a senior civil servant said.
Exchequer ministers' post bags are being filled with questions about what is going on, added Dr Andrew McCormick, the permanent secretary in Stormont's economy department.
The total cost of the Renewable Heat Incentive (RHI) scheme is projected to be more than £1 billion over the next 20 years.
While the Treasury will pick up the bill for most of it, an expected overspend of around £490 million will not be covered and will come out of the block grant for public services in Northern Ireland.
Dr McCormick said: "They read the Northern Ireland newspapers, they have noticed this and are very concerned and there is a bit of a post bag into their ministers to say 'what is going on here'.
"Given the contribution from UK taxpayers to Northern Ireland there is concern in London and we do have de facto a responsibility to act."
He said he had not received a warning letter from the Treasury but added: "We are aware of concerns informally".
Dr McCormick was giving evidence to a Stormont committee.
The committee also heard evidence from those who availed of the scheme.
Elaine Shaw, from the Renewable Heat Association, represented the mushroom industry and said it had reduced growing costs.
There were warnings any reduction in the rate paid could force businesses to go back to burning fossil fuels.
John Martin, from the association, criticised proposals to mitigate the cost being considered by Stormont ministers.
He added: "All of the focus has been on the negative, none on the positive.
"We believe that the rush to draft these amendments has bypassed the scrutiny mechanism that should be the cornerstone of good government."
