Downing Street has refused to rule out changes to the 48-hour working week amid reports that it could be scrapped under plans for a potential post-Brexit overhaul of labour laws.

Boris Johnson’s spokesperson insisted “we will not be lowering workers’ rights” but said the government was looking at policies to “stimulate business growth”.

Asked to rule out changes to the working time directive, they replied: “We will continue to protect and enhance workers’ rights, we will continue to look at policies to help and stimulate business growth, innovation and job creation.

“But those policies would never be at the expense of workers’ rights.”

It comes after the Financial Times reported that the 48-hour week was among worker protections in EU law that the business department had included in a package of deregulation for a possible post-Brexit shake up.

No decisions have been made by ministers and government figures have insisted they will not lower workers rights.

But asked if other reported options such as removing overtime form holiday entitlement and changing rules around breaks count as reducing workers’ rights in the eyes of the prime minister, the No.10 spokesperson said: “I’m not going to get into hypotheticals but the general principle remains the case that we’ve got one of the best workers’ rights records in the world, this government is not going to do anything to change that.

“We will not be lowering workers’ rights.”

Labour described the reported plan as a “disgrace” and warned it would fight any such moves “tooth and nail”.

But new business secretary Kwasi Kwarteng insisted the government wanted to improve workplace protections – not reduce them.

In 2012, Kwarteng authored the pro-free market book Britannia Unchained alongside fellow cabinet members Priti Patel, Dominic Raab and Liz Truss, which accused British workers of being “among the worst idlers in the world”.

He said: “We are not going to lower the standards of workers’ rights,” he tweeted.

“The UK has one of the best workers’ rights records in the world – going further than the EU in many areas.

“We want to protect and enhance workers’ rights going forward, not row back on them.”

The Department for Business, Energy and Industrial Strategy said labour market policy was kept under “regular review” to ensure businesses had the “appropriate freedoms and flexibility to innovate and grow” while safeguarding protections for workers.

It said in the 2019 Queen’s Speech, the government committed to introduce an employment bill to strike “the right balance between the flexibility that the economy needs and the security that workers deserve”.

Shadow business secretary Ed Miliband said Labour would strongly oppose any moves to reduce existing standards.

“These proposals are not about cutting red tape for businesses but ripping up vital rights for workers. They should not even be up for discussion,” he said.

“People are already deeply worried about their jobs and health. It’s a disgrace the government is considering forcing them to work longer hours or lose paid holidays.”