Almost half of GPs have admitted prescribing antibiotics even though they know they will not treat the patient's condition, a survey shows.

Ninety per cent of doctors who responded to a recent questionnaire said they felt pressure from patients to hand out the medications.

The Longitude Prize survey compiled responses from 1,004 GPs across the UK, and found 28% have prescribed antibiotics several times a week even when they are not sure if it is medically necessary, and 45% have done so knowing they will not help.

The findings come after experts at Public Health England and University College London revealed earlier this month that the number of patients given antibiotics for minor ailments has soared in recent years.

While 36% of patients were given antibiotics for coughs and colds in 1999, by 2011 this figure had soared to 51%, the study published in the Journal of Antimicrobial Chemotherapy found.

Last month, Prime Minister David Cameron said that resistance to antibiotics was a "very real and worrying threat" as he pledged to put Britain at the forefront of the fight against drug-immune bacteria threatening to send medicine "back to the dark ages".

The latest research comes ahead of the £10 million Longitude Prize which will open later this year for entries to find an easy and cost-effective test for bacterial infections that doctors can use to determine if and when to give out antibiotics.

Seventy per cent of GPs surveyed said they prescribe because they are not sure whether the patient has a viral or bacterial infection, and 24% say it is because there is a lack of easy-to-use diagnostic tools.

Dr Rosemary Leonard said she understands the pressures GPs are faced with to prescribe antibiotics when they are not actually necessary.

"The more antibiotics taken, the more resistant bacteria come to them," said Dr Leonard. "Antibiotic resistance is a real issue and more needs to be done to conserve antibiotics for the future.

"Diagnostics play a valuable role in making this happen. Not only can diagnostics help determine the type of infection someone has, they could gather valuable data and aid the global surveillance efforts."

Tamar Ghosh who leads the Longitude Prize, said accurate diagnostic tools can help curb the unnecessary use of antibiotics across the globe.

"In the next five years, the Longitude Prize aims to find a cheap and effective diagnostic tool that can be used anywhere in the world," said Ms Ghosh.

"We recognise that stemming the misuse and overuse of antibiotics is just one piece of the jigsaw to slow bacterial resistance to antibiotics.

"Nevertheless it's an important step when we could be waiting many years for other solutions, including novel alternatives to antibiotics coming to the market."

Just 6% of 1,074 patients surveyed by Populus earlier this month said they would push their GPs to give them antibiotics, despite the high number of doctors saying they feel under pressure to do so.