Mortgage borrowers today faced long delays and the risk of being pushed on to higher rates because lenders are having to deal with huge back-logs of loan applications.

Brokers said that borrowers who are applying for one of the few remaining good value mortgage deals could miss out as lenders reserve the right to pull offers without notice.

A dramatic decline in the number of mortgages and a rise in the cost of deals has caused a surge in demand that brokers and lenders are struggling to meet.

Today, Woolwich, the mortgage arm of Barclays, increased rates on its lifetime trackers by between 0.55 per cent and 0.70 per cent, depending on the size of the loan.

C&G, owned by Lloyds TSB, said that it would reduce the amount it is willing to lend borrowers with £1 million-plus loans from 90 per cent to 80 per cent. Skipton Building Society pulled its three-year fixed rate mortgage and two of its tracker products, while Derbyshire Building Society is withdrawing its entire tracker range temporarily.

Loughborough Building Society said it would no longer lend products through brokers, while West Bromwich Building Society pulled all of its broker-only deals.

The brokers said that anyone who only has an "agreement in principle" with their mortgage lender and does not apply immediately for the loan faces "a very strong likelihood" that the rate will no longer be available the following day and will have been replaced by a much more expensive mortgage.

Rob Clifford, the chief executive of Mortgageforce, the broker, said: "Can you rely upon a quote from a lender? The answer is no. Lenders are not obliged to honour quotes and could elect to reject all applications or offer them less attractive deals. More typically, lenders close the doors to new applications, making the quote invalid, but tend to honour applications already received. However that may change. The fact remains that consumers haven't got a cast iron deal, unless they have received a formal mortgage offer from the lender — which is then binding."

David Hollingworth, of London & Country, the mortgage broker, said: "The changing market makes life a lot more difficult, it is a challenge to keep on top of all the changes. We are working extended hours to get applications through before rates are withdrawn.

"Whereas borrowers used to be able to go away and mull over a mortgage offer before deciding to apply, the danger is that now they cannot afford to do that, as a rate available on Friday could be gone by Monday."

In theory, lenders can pull an offer at any time until the money is actually in the solicitor's account. Lenders treat the various stages of the application process differently.

Melanie Bien, of Savills Private Finance, another broker, said: "A 'decision in principle' may be considered by some as reserving the rate but it is subject to underwriting criteria so if you fail this, you won't get the mortgage. Other lenders may consider it less binding than this."

Mortgage offers are honoured for three to six months after they are made. The contracts are then exchanged.

However the amount of time it takes to receive a mortgage offer is now "a lot longer" than the usual two week period, Mr Hollingworth said, with borrowers facing indefinite delays.