The British government’s estimate of how much it will have to pay the European Union as part of its so-called divorce settlement is at least £10 billion ($13 billion) too low, a committee of lawmakers said on Wednesday.

Negotiators in London and Brussels have agreed on a divorce bill of £35 billion to £39 billion, due to be paid over the next few decades after Britain leaves the bloc.

The bill was one of the most inflammatory elements of Britain’s withdrawal negotiations, with vocal Brexit campaigners in Prime Minister Theresa May’s party angry at having to pay anything at all. The agreement was seen as an achievement for May because it came in lower than some had initially feared.

But parliament’s Public Accounts Committee said the figure, which estimated the cost to the country as a whole, was an underestimate of the actual cost to public finances and said the government needed to be clearer.

“The true cost of Brexit is a matter of outstanding public interest. Government must provide parliament and the public with clear and unambiguous information,” committee chairwoman Meg Hillier said.

“Government’s narrow estimate of the so-called divorce bill does not meet this description. It omits at least £10 billion of anticipated costs associated with EU withdrawal and remains subject to many uncertainties,” she added.

The report said the current figure did not include £3 billion of payments Britain would have to make to the European Development Fund, which the EU uses to provide overseas aid.

The finance ministry said May had been clear that Britain would honor commitments to the EU which were made while it was still a member of the bloc.

“We have negotiated a settlement that is fair to U.K. taxpayers and ensures we will not pay for any additional EU spending beyond what we signed up to as a member,” a Treasury spokesman said.

The committee also said the estimated overall settlement included around £7.2 billion of EU funding which will go directly to private sector bodies and would therefore not offset the cost to government.

The original finance ministry estimate of the overall cost to the country did not distinguish between flows to private and public sector bodies, and simply deducted a combined amount from the cost.

The Treasury said: “The National Audit Office confirmed in April that our estimated figure is a reasonable calculation. Now we are discussing what our future relationship looks like.”

In a speech on Wednesday, former Prime Minister Tony Blair was to say that the U.K. should start planning to delay Brexit and extend negotiations with the European Union or face the disaster of leaving without a deal.

In an impassioned appeal for a change of course, Blair was to warn that crashing out of the EU with no agreement will be “devastating” for the U.K., declaring himself more worried than ever about the country’s future.

Blair believes May is allowing Britain to drift aimlessly toward exit day because she is so weak as to be “more a hostage than a leader.”

“We cannot go on like this,” Blair was to tell the Chatham House think tank in London on Wednesday, according to extracts released in advance by his office. “We should plan now for the possibility we need to extend the March 2019 deadline.”

Blair’s intervention will deepen the divisions within the government over May’s handling of Brexit at a critical time as she prepares to finalize her list of negotiating demands.

In his speech, he also was to heap pressure on the main opposition Labour Party — which he used to lead — accusing it of failing to play its role in saving the country from ruin.

“Parliament must assert itself because neither government nor opposition can or will,” Blair was to say. “Then the people must make the final decision because only they have the right to decide what version of Brexit they want or whether in the light of all they now know they prefer to remain.”

May is heading into an EU leaders’ summit on Thursday at which her European counterparts are set to rebuke her over the slow progress of the negotiations. The U.K. is due to exit the EU on March 29, but talks are stuck on the terms of the divorce and no serious discussions have started on the kind of trade arrangement the two sides will have in future.

May is struggling to build a consensus within her Cabinet on what kind of customs and trade model to propose to the EU. She has promised more detail in a 150-page plan to be published next month but must first win the support of her most senior ministers.

In his speech, Blair voices his fear that the rise of populism in the West poses a real threat to democracy, raising the specter of a return to the kind of fascist politics that Europe witnessed in the 1930s.

“The populist wave upending Western politics shows no sign of abating,” he will say. “It is difficult to predict whether we are at the crest of the wave which will soon subside or whether it is still building its momentum. But I fear it is the latter.”