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U.K. Chancellor of the Exchequer Philip Hammond found himself on the defensive after his budget decision to break a 2015 election pledge and increase a tax on income paid by the self-employed.

The chancellor woke to universally critical headlines about the hike in the national-insurance contributions rate. According to the Treasury, the $292 a year, though Hammond pointed out that a cut elsewhere in the system announced by his predecessor, George Osborne, means that 60 percent of self-employed workers will be better off overall.

The move was criticized by Tory lawmakers not simply because it was a tax rise on entrepreneurs, but because it breaks a promise from the last election that the Conservatives wouldn't increase income tax or national insurance.

Hammond argued that this pledge had been a "broad commitment" that only covered the main national-insurance rate.

More serious for Hammond was the number of Conservatives publicly criticizing his decision. May's government has a working majority of 17, meaning it can lose a vote if as few as nine Tories rebel.

Tory lawmaker Anne-Marie Trevelyan said the tax increase "goes against every principle of Conservative understanding of business." Her colleague, Anna Soubry, predicted opposition to the move could lead to the "first U-turn" of the Budget.

"It hurts entrepreneurs and it breaks an election promise. He would have struggled to pick something more likely to provoke a Tory rebellion," said Philip Cowley, professor of politics at Queen Mary University of London and an expert on parliamentary revolts. "This is a government with a tiny majority. Let's see if it makes it."

The increase was the most contentious part of a fiscally restrained budget that sought to prepare the economy for leaving the European Union with investments in schools and technical education. Prime Minister Theresa May has pledged to trigger formal negotiations this month.

"Britain's circumstances have moved on," Hammond told ITV television on Thursday. "We're now facing the challenge of leaving the European Union, of building a global Britain to exploit the opportunities in the future."

Hammond defended the decision to target the self-employed on the grounds of fairness, saying a person earning 32,000 pounds a year pays only a third of the tax levied on employees and the companies they work for.

That, he argued, is no longer justifiable now that the self-employed enjoy similar pension entitlements to employees. People who work for themselves now account for 15 percent of the British workforce.

The knock-on effect of the budget has been to quash speculation that May will call an election in the coming months.

The prime minister had been facing growing calls to make the most of her poll ratings and an opposition in disarray by holding an early general election to bolster her slim parliamentary majority.

While another European election would add to political risk, victory for May would strengthen her hand as she enters divorce talks with the EU, according to Conservatives including William Hague, a one-time party leader and former foreign secretary.

"Think we can rule out a snap election," Rupert Harrison, a former aide to Hammond's predecessor, George Osborne, and now chief macro strategist at BlackRock, said on Twitter.

The planned increase in the Class 4 NIC rate to 11 percent from 9 percent by 2019 will raise about 2.1 billion pounds over the next five years, according to Treasury costings.

It applies to self-employed workers with annual profits of more than 8,060 pounds. Osborne abolished Class 2 contributions, a flat-rate levy for those with low earnings.

A separate decision to lower the tax-free threshold for dividend payments will generate about 2.6 billion pounds by 2022, potentially putting the government on a collision course with Tory-supporting savers, pensioners and business owners.

Together, the revenue will help fund extra spending on social care, education and relief for those hardest-hit by changes to a tax on commercial premises.