HM Revenue & Customs has stepped up its investigation into the delivery company Hermes classifiying its couriers as self-employed, while the business has also been hit with an employment rights lawsuit from the GMB trade union.

Drivers for Hermes were sent letters from HMRC over the weekend asking them to provide evidence as the tax authority looks into their employment status.

In the letter, seen by the Guardian, HMRC requests that the drivers disclose information such as their written contract and payslips, and agree to a one-hour interview. “This will help us decide what your employment status is/was,” it says.

HMRC’s investigation follows one by the Guardian that found some self-employed couriers were being paid less than the “national living wage”, in an arrangement the company said had been approved by HMRC.

Separately, GMB has filed a lawsuit challenging Hermes over employment conditions for its couriers, vowing to battle “bogus self-employment and gig economy exploitation”.

Maria Ludkin, the GMB’s legal director, said: “Under the false claims of ‘flexibility’, Hermes seems to think it’s acceptable to wriggle out of treating its workers with respect.

“Guaranteed hours, sick pay, pension contributions – these aren’t privileges to be bestowed when companies feel like it; they are the legal right of all UK workers.

“It’s good to hear that HMRC are finally taking their enforcement responsibilities seriously and are investigating Hermes.”

The union won a similar case against Uber last year, resulting in a ruling that the ride-hailing service should pay the minimum wage and grant drivers holiday pay and other benefits. Uber is contesting the ruling at the employment appeal tribunal.

The case against Hermes, lodged with the employment tribunal by law firm Leigh Day, opens a new front in unions’ efforts to address the precariousness of gig economy labour.

Michael Newman of Leigh Day said: “We believe that Hermes are deliberately avoiding giving their couriers the rights to which they are entitled. They do so by labelling the couriers who work for them as self-employed, when the reality is different.”

Hermes had not responded to a request for comment at the time of publication.

The growth of the gig economy has given rise to a series of legal disputes between workers and companies that seek to keep costs down by classifying them as something other than full employees.

As well as Uber and Hermes, takeaway food delivery service Deliveroo has come under pressure from workers seeking improved conditions.

The company was accused of “creating vocabulary” last month, after issuing managers with a list of words to ensure it did not accidentally use terms that indicated its motorbike riders and cyclists were employees.

Successive disputes over labour rights have also seen the work and pensions select committee launch an inquiry into the gig economy.

The committee, led by Labour’s Frank Field, has accused companies of writing contracts in “gibberish”, making it hard for workers, particularly those with limited English skills, to understand the terms on which they are signing up.

On May Day, designated International Workers’ Day in many countries, the committee called on the government to tighten up laws to prevent the exploitation of workers.

GMB said it would continue looking into “contracts masquerading as bogus self-employment”.