The announcement followed talks between the Public and Commercial Services Union and Home Office Minister Mike O'Brien on Monday.

The Passport Agency has a backlog of 530,000 applications - half of them for tourists expecting to travel in August and September.

The average waiting time for passports, according to the unions, is now more than seven weeks, compared to a target time of two weeks.

The problem is partially due to a new law requiring children to have separate passports which has led to a larger-than-expected rise in applications.

Computer problems

The introduction of a new computer system at two of its offices has not helped either.

A union spokesman said the meeting between their national officer for passports, Frank Campbell, and Mr O'Brien had been "very helpful".

High profile campaign

The spokesman said: "He was very sympathetic to the points we were making and said he would discuss them with management.

"He is committed to high profile advertising to passport applicants to let them know what's going on, which should deal with the queues at passport offices, by letting them know that if they are travelling further in advance, they don't need to come and queue."

Many want to travel in August and September

Mr O'Brien also revealed, in a Commons written reply, that nearly 19,000 extensions were granted in May and more than 19,100 so far this month.

Only 183 extensions had been granted previously in the last three years in emergency cases.

Loss of income

Mr O'Brien said the two-year extensions were likely to cost the Passport Agency over £1m in lost income, but he said this would be recouped in two years.

The Passport Agency and the Home Office have promised compensation payments to people who miss their holidays because of the delays but the cost will be met from the fees paid by other travellers.

Extra staff

An extra 300 new staff have been taken on to cope with the backlog as the agency attempts to process twice as many passport applications as this time last year.

But queues in Glasgow, Liverpool and London remained hundreds deep on Monday as people attempted to get their passports ahead of their holidays.

Call for seven day-week

There has been criticism that passport offices are not open seven days a week.

Union joint general secretary Barry Reamsbottom said: "The staff at the Passport Agency are already working every evening and every weekend.

"Opening the public counters at weekends would drag staff away from dealing with the backlog and just make the queues worse."

The agency has said it has already processed 2.4 million passports so far this year, with just 50 holiday dates missed.

MPs will discuss the government's handling of the situation in an opposition day debate on Tuesday.