The largest businesses in Britain were shocked by Labour's proposal to increase National Insurance contributions, and now they face a £1 billion tax bombshell.The latest tax hike will result in hundreds of millions of pounds in additional tax that some of the biggest companies, which collectively employ over a million people, will owe the Treasury. With a workforce of around 75,000 employees since its arrival in the UK in 1998, online retailer Amazon is one of the largest employers in the UK.Amazon, one of the UK's biggest taxpayers, now faces an extra £110 million being added to its annual bill.

Similarly, Royal Mail, which employs an army of more than 130,000 posties, is estimated to see an increase of £110 million, piling further pressure on the business, which has struggled with large losses in recent years.It comes as an ever-growing list of business leaders have lined up to lambast Chancellor Rachel Reeves for her decision last month to raise the National Insurance rate paid by employers on staff wages from 13.8 per cent to 15 per cent and cut the threshold at which firms start paying it from £9,100 to £5,000 a year.The Treasury hopes the measures will raise £25 billion a year. But they have sparked alarm from employers and warnings it could hit wage growth and push up prices. Marks & Spencer boss Stuart Machin has called the hikes a 'double whammy' and the head of Sainsbury's, Simon Roberts, said the 'barrage of costs' could reignite inflation.In total, some of the UK's biggest supermarkets – Tesco, Asda, Sainsbury's, Morrisons and M&S – are facing a £650 million increase in their costs. Retailers are not the only sector suffering from the Chancellor's tax hike.

Allison Kirkby, the boss of telecoms giant BT, revealed the firm would take a £100 million hit from the changes and that prices may have to rise to offset the damage.She added that the company would accelerate cost-cutting efforts, including job losses.

'We will be going harder and faster on the cost transformation and plans we have laid out,' Kirkby said previously.

Publicans have also sounded the alarm over the changes, saying tax hikes risk making trips to the pub even more expensive.

Simon Dodd, boss of pub operator Young's, warned that staff hours will be cut and the price of a pint will rise as it adjusted to the effects of the Budget.