A care company exposed by the Guardian for fining staff £50 when they phone in sick should scrap the “absolutely disgusting” charge immediately, the shadow health secretary has said.

The intervention by Jonathan Ashworth came after an investigation revealed Newcross Healthcare Solutions, which made millions of pounds in profit last year, was imposing the charge on workers who called to say they were unwell without giving 24 hours’ notice.

The company, which provides thousands of agency nurses and care workers for homes and hospitals across the country, announced the charge would be “phased out” by April 2019, but Ashworth called for it to be ditched without hesitation.

“Bosses at this company should scrap these £50 fines now, today, not ‘by April 2019’,” he tweeted.

Ashworth later said: “Every decent-minded person will be absolutely disgusted at the way this firm treats its staff. Care workers look after our elderly relatives, going way beyond the call of duty. These £50 fines should be ended now.”

Meanwhile, it has emerged that Newcross’s founder and chief executive may be a Conservative party donor.

According to Electoral Commission records, a Stephen Pattrick has given a total of £15,000 to the Conservatives, making one donation of £7,500 in November 2008 and another of £7,500 in December 2009.

Newcross declined to comment. A Conservative spokeswoman would not confirm the donor was the same Stephen Pattrick, but said the party “maintains a system of due diligence for all our donors”.

Pattrick, who posted pictures on Facebook of him smoking cigars and shooting, is based in the tax haven of Gibraltar and is also chief executive of the motorsports team Bullitt Racing, for which he has driven.

The team’s website said his success in business had enabled him to “indulge in his motor racing hobby”. However, after the Guardian published its investigation, Pattrick’s Facebook page was deleted and Bullitt’s website was cleared of content about its founder.

Newcross, which made £21m in pre-tax profit and paid directors £17m in dividends in 2017, denied it imposed the charge when workers were “genuinely ill”, but the Guardian found dozens of examples that suggested otherwise.

Former and current Newcross workers have contacted the Guardian to speak out about the charge. They include an ex-employee who claimed he had been fined £50 for cancelling a shift he could not make because of a family emergency.

The individual, who worked for the company for four years until 2016, accused it of “ripping workers off”, saying he quit because Newcross allegedly charged him £50 by mistake for cancelling a shift on another occasion when he gave more than 24 hours’ notice. He ended up “chasing” the company for two months before it refunded the money, the former employee said.

“Newcross have been ripping workers off of this £50 for so long. I had so many arguments with Newcross about it. I’m saying, ‘Why can’t you be reasonable and listen to people’s reasons why they cancel the shifts?’” he said.

“They will not listen to any explanation, or any reasoning, and they will take their £50.”

He added that it was unfair the company charged workers for cancelling shifts when it routinely did the same to workers’ shifts at short notice.

A 10-year veteran of the care industry, who has worked as a healthcare assistant for Newcross for more than two years, criticised the charge and said staff were “disgruntled”. He said he had to pay £25 for his uniform and regularly had to pay up to £30 a time for training. “I’ve just had enough, it’s a joke really,” the worker added.

Newcross declined to comment.