The chancellor is expected to use her upcoming budget to impose a new levy on people who use limited liability partnerships (LLPs), which allow members to avoid employer national insurance contributions. Partners are treated as self-employed, meaning firms are exempt from the 15 per cent employer rate.

Ms Reeves, who has repeatedly said that “those with the broadest shoulders” should pay their “fair share of tax”, believes this treatment is unfair. Economists estimate that a solicitor earning the average partnership profit of £316,000 would face a new charge of about £23,000 – equivalent to a 7.3 per cent rate.

More than 190,000 people work under LLPs, including 13,000 partners earning an average of £1.25 million a year. Solicitors make an average of £316,000. The Centre for the Analysis of Taxation (CenTax) says solicitors receive a fifth of all partnership income.

The new levy, which Ms Reeves is expected to frame as a measure to “equalise the tax treatment”, would apply at a slightly lower rate than employer national insurance. It will form part of a broader package targeting the wealthy, including a possible “mansion tax” that would bring expensive homes within the scope of capital gains tax.

Arun Advani, director of CenTax, said: “Like so much of the UK tax system, the absence of employer NICs [national insurance contributions] on partners comes from a sequence of small accidents, rather than any deliberate design. It has only survived because, like the non-dom regime, it was never costed or listed officially as a relief, preventing parliamentary and media scrutiny.

“Since partnership income is hugely concentrated, with almost half going to those in the top 0.1 per cent, exempting partners from any equivalent to employer NICs is very regressive and simply means higher taxes for everyone else. There is no reason partners should pay less tax than similarly highly paid employees and business owners.”

Stuart Adam, a senior economist at the Institute for Fiscal Studies, added: “People in LLPs are generally very well off and in many cases are just supplying their labour like ordinary employees are, so it’s not clear why they should get preferential treatment. But as with any tax rise, it can provide a disincentive to work. There may be other ways that people might respond, including in the most extreme cases leaving the UK or not coming in the first place.

“More broadly, this is just picking out one particular group of the self-employed. It doesn’t address the more fundamental issue of how we tax the self-employed compared to employees. It just moves the problematic boundary.”