Millions of people will be able to get sick pay from the first day they are ill and claim unpaid parental leave as soon as they start a job, in a major overhaul of workers’ rights.

However, certain measures in the Employment Rights Bill have been criticised for being watered down or delayed.

Most of the planned changes will not take effect for two years following a period of consultation.

The government said the bill marked the “biggest upgrade to rights at work for a generation”, but business groups are concerned about how the changes will work in practice.

Deputy Prime Minister Angela Rayner defended the time taken for changes to be introduced.

“Some things will take a little bit longer,” she said, because of the scale of the reforms.

But she said many employers had already brought in some of the proposed changes.

The government is seeking to be pro-worker and pro-business and striking that balance means that much of the detail is still to be decided.

As part of the plans, the existing two-year qualifying period for protections from unfair dismissal will be removed.

Ministers have said this will benefit about nine million workers who have been with their current employer for less than two years.

However, workers will be subject to a proposed nine-month probation period when they can still be sacked without a full process.

If a worker is sacked within those nine months, it is unclear under what circumstances they can make a claim for unfair dismissal and when they cannot.

• Statutory sick pay (SSP): Workers will be entitled to SSP from the first day they are ill, rather than the fourth day
• Lower earnings limit for SSP: Currently, workers earning less than £123 per week cannot claim SSP. This limit will be removed but the bill will set out a lesser level of sick pay for lower earners
• Paternity leave: Fathers or partners to be eligible from day one of employment, instead of 26 weeks
• Unpaid parental leave: Parents to be eligible from day one of employment, instead of one year
• Unpaid bereavement leave: To become a “day one” right for workers
• Flexible working: Bosses will be expected to consider any flexible working requests made from day one, and say yes unless they can prove it is unreasonable. There are eight grounds they could give for refusing a request, relating to impact on the business

Many big business groups welcomed the plans, with the CBI saying the government deserved credit for engaging with employers and unions when working on the bill.

Ann Francke of the Chartered Management Institute, said that for many firms the proposed changes “won’t create much of a challenge, as the government is in many cases just formalising practices that smart employers already follow”.

However, while that may be the case for big businesses, smaller firms may be left “scrabbling to make sense” of the changes, according to Tina McKenzie, policy chair at the Federation of Small Businesses, whose members employ up to 250 workers.

She said the new bill was a “rushed job, clumsy, chaotic and poorly planned”.

The government will also consult on a new probation period for new hires, with a recommendation of nine months, despite a previous suggestion of six months.

It proved a tricky subject during discussions, with businesses pressing for longer and unions shorter.

Dominic Ponniah, the boss of Cleanology, told the BBC his firm is delaying hiring plans and being more cautious of who it takes on.

His cleaning company has about 1,300 employees located from Scotland to Southampton.

“It’s just another thing that businesses have to contend with,” he said, adding that the new rules around sick pay, unfair dismissal and probationary periods would make businesses “very, very nervous”.

The matter of zero-hours contracts has also been hotly debated.

Under the new bill, workers on such contracts will have to be offered a guaranteed-hours contract based on the hours they have clocked up during a 12-week period, and will be entitled to “reasonable” notice of any shift changes, as well as compensation if a shift is cancelled or cut short.

Tamara Beattie works for a healthcare agency and has been on a zero-hours contract for more than two years since having her daughter.

“I don’t know how [many] hours I’m going to get each month so I have to be careful when spending,” says Tamara, who is a GMB union representative.

While she’s glad of the change, she says the fact it is taking two years to come into effect is “a wee bit poor” and that something that needs to be implemented now.

Rachel Harrison, national secretary of the GMB, called the bill a “groundbreaking first step” in improving workers rights, but added there was still a “long way to go”.

Some measures included in Labour’s plan to “Make Work Pay”, issued in the run-up to the general election, will not feature in the bill either.

The “right to switch off” stopping employers contacting staff out of hours, for example, will be addressed separately.

Conservative shadow business secretary Kevin Hollinrake said the party would look closely at the detail of what Labour has set out.

“But businesses and the economy needs certainty, not the threat of being sent back to the 1970s, unleashing waves of low-threshold, zero-warning strikes, driving down growth and slowing productivity,” he said.