Furious Amazon Prime customers have complained after being forced to wait four days for deliveries when the service promises to have packages at your door in 48 hours.

The online goods giant has received several complaints from customers who have signed up for its premium service Prime, which costs £79 a year, only to be left disappointed in the run up to Christmas.

Amazon has recently been dogged by claims it threatens its drivers with £110 fines if they call in sick.

The complaints about the missed delivery targets are now being considered by the advertising watchdog, who received 'a handful' of reports from people who had signed up to Prime.

The advertising watchdog is considering complaints against Amazon for misleading customers on delivery times amid claims it threatens drivers with £110 fines if they call in sick

A spokesman for the Advertising Standards Authority told the Independent the complaints were being 'considered', but added that no official investigation had been opened at this stage.

Amazon had previously told customers delivery times would vary because it is a busy time of year.

Chris Styles, from the West Midlands, was expecting a present for his niece to be delivered in one day.

He told the Sunday Telegraph: 'I spent all day on the phone with Amazon. My orders were due on Friday and were guaranteed for 8pm and have still not arrived. I pay about £80 a year for Prime and this is about the third or fourth time it has happened.'

Amazon's help desk said: 'This is a busy time of year and due to high order volumes, estimated delivery dates can vary.'

A spokesman added: 'We'd ask any customers with questions about their deliveries to contact us.'

It comes as some drivers claim they were forced to be available seven days a week and threatened with fines if they were ill, the Sunday Mirror reports.

At least four drivers, who worked for Amazon transportation subcontractor UK Express, have brought claims to an employment tribunal that they were denied basic rights.

The company has several countless complaints for failing to deliver items on time in the run up to Christmas

A £79-a-year Amazon Prime subscription promises customers delivery within two days but many have found items taking up to four days to arrive

A spokesman for Amazon said: 'We cannot comment on employment tribunal proceedings involving claims against an independent delivery company.

Our delivery providers must abide by Amazon's Supplier Code of Conduct and we investigate any allegation that a delivery provider is not meeting our expectations.

It was claimed last month that Amazon's staff are falling asleep on their feet and being taken away in ambulances as they struggle to meet warehouse targets.

Cameras monitor every move as employees try to process up to 300 items an hour, it has been alleged. Screens remind them if they are falling short.

Exhausted staff are said to cover clocks so they are not reminded how long there is to go on their shifts, and have to walk up to a third of a mile to use the toilet.

The claims in a newspaper were made about the online retailer's newest warehouse – which the company refers to as a 'fulfilment centre' – in Tilbury, Essex.

The packing plant is the biggest in Europe, the size of 11 football pitches, and is due to ship 1.2million items this year.

Amazon's staff are falling asleep on their feet because of extremely tough working conditions

Amazon workers were seen asleep on the job as they complained about intolerable working conditions at the firm's Tilbury warehouse

But the investigation, by an undercover reporter for the Sunday Mirror who spent five weeks there, suggested workers suffer mentally and physically as they try to meet demand.

He said that some of his colleagues were so tired from working 55-hour weeks that they would 'sleep on their feet'.

'Those who could not keep up with the punishing targets faced the sack – and some who buckled under the strain had to be attended by ambulance crews,' he added.

It comes at a time when Amazon has promised to improve the treatment of staff following accusations of poor conditions and low pay.

Workers in Italy and Germany have gone on strike in protest at their workloads.

But one worker in Tilbury reportedly complained: 'At my induction, someone was asking why the staff turnover was so high. It's because they're killing people. All my friends think I'm dead. I'm exhausted.'

Staff complained about the state of the toilet in the Tilbury warehouse in Essex (pictured)

Another is said to have written on a whiteboard for staff comments: 'Why are we not allowed to sit when it is quiet and not busy? We are human beings, not slaves and animals.'

Amazon said yesterday: 'Amazon provides a safe and positive workplace with competitive pay and benefits. We are proud to have created thousands of roles in our UK fulfilment centres. As with most companies, we expect a certain level of performance.

'Targets are based on previous performance achieved by our workers. Associates are evaluated over a long period of time.'

The retailer made £7.3billion of sales in the UK last year and employs 24,000 people.

Staff who buckle under the pressure are taken away in ambulances, it was claimed

The Essex plant has no natural light, so staff cannot see whether it is day or night outside. The undercover reporter worked from 7.30am to 6pm with two half-hour breaks, giving just enough time to race to the canteen for food.

One colleague is said to have collapsed as they worked and was taken to hospital by ambulance. Another was seen by paramedics after suffering a panic attack when she learned compulsory overtime meant she would have to work up to 55 hours a week over Christmas.

One worker told the paper: 'Everybody suffers here.' They added that staff reported a variety of stress-related injuries, including pulled hamstrings and ligaments.

Once items are picked they go to a packing area where workers prepare 120 items for shipping every hour – rumoured to be increasing to 200 – or 85 multiple items, according to the investigation.

This board showed some of the staff's grievances at the Essex warehouse, including complaints about the 'disgusting' toilet

Shifts begin at 7.30 in the morning and finish at 6pm, with workers given just two half-hour breaks during the day

Many patrolled conveyor belts looking for smaller items to keep up their average.

Amazon has also defended itself after it was revealed that ambulances were sent to its main Scottish warehouse 43 times last year.

Emergencies included 15 of the most serious Category A classification, with 23 workers taken to nearby hospitals. Call-outs to the depot in Dunfermline involved two staff being treated for falls, a call about 'traumatic injuries' and two for 'industrial accidents'.

An Amazon spokesman said: 'Independent safety experts certify our compliance with all relevant laws and regulations.'