LONDON — British MPs have urged the U.K.’s spending watchdog to investigate what the U.K.-U.S. drug-pricing deal will actually cost taxpayers, after the government refused to publish full figures.

Six parliamentary committee chairs wrote to the National Audit Office on Wednesday, saying there is “significant public interest in understanding its implications for public expenditure,” and asking for a rapid investigation into whether the arrangement represents value for money for the public. The lawmakers wrote.

The deal, struck in December under the Trump administration, commits the U.K. to higher spending on new medicines in exchange for three years of zero tariffs from the U.S. on pharmaceutical products. Critics say the haste and secrecy around the agreement raise questions about the U.K.’s bargaining position and undue influence from Washington.

As part of the arrangement the government agreed to raise pharma spending to 0.6% of GDP by 2035 and laid secondary legislation to let the National Institute for Health and Care Excellence increase its cost-effectiveness threshold by 25% — effectively allowing pricier treatments to be judged acceptable.

“Given the potentially significant financial implications of this agreement … we would like to formally request that the National Audit Office undertake a rapid investigation into the costs of the arrangement and assess whether it represents value for money for the taxpayer,” the letter says.

Without a full impact assessment covering future years, parliament and the public cannot properly judge the long-term effect on NHS budgets and wider public spending, the chairs add.

So far the government has refused to publish the full cost to the NHS, offering only an estimate of around £1 billion over the current spending review period.

Sally Gainsbury, senior policy analyst at independent health think tank Nuffield Trust, has been refused access to the government’s impact assessment of the deal. The Information Commissioner’s Office said the Department of Health and Social Care argued that releasing it would “prejudice ongoing policy development, international relations and commercial interests.”

Gainsbury launched an appeal to the Information Tribunal on Aug. 17 over the decision to withhold the assessment. The case is due to be heard between December and February.

Gainsbury warned the trade terms could “divert resources away from much more cost-effective forms of care and towards much more expensive and often less certain treatments,” potentially harming the NHS’s ability to improve public health.

She added that the government claims the deal is designed to support economic growth but has not shared the evidence behind that claim. “We shouldn’t have to go to court to get transparency on something so significant and impactful on our nation’s health services, but that is where we find ourselves,” she said.

The letter was signed by: Health and Social Care Committee Chair Layla Moran; Science, Innovation and Technology Committee Chair Chi Onwurah; Business and Trade Committee Chair Liam Byrne; Northern Ireland Affairs Committee Chair Tonia Antoniazzi; Scottish Affairs Committee Chair Patricia Ferguson; and Welsh Affairs Committee Chair Ruth Jones.