Deals are in danger of collapsing or racking up huge costs for buyers because lenders are reassessing mortgage offers after buyers and sellers have exchanged.

Peter Ambrose, who heads up major conveyancing firm The Partnership, said that in two cases, Halifax has stepped in to prevent completion taking place on the dates agreed.

He said that in all his years of experience, he has never known this before.

He added: “It is a bit of a shocker. If this is happening to us, it will be happening to others.”

Ambrose said that the lender is using the Mortgage Market Review as the reason. Under MMR, implemented at the end of April, lenders must ask applicants far more searching questions as to their spending habits and their ability to pay a mortgage should interest rates rise or their circumstances change.

Ambrose said that in the two cases his firm has been handling, both sets of buyers were given mortgage approvals months ago – well before MMR.

He said: “We were also assured that there would be no retrospective use of MMR. However, our experience is proving different.”

In one case two weeks ago, he said, completion was halted after Halifax refused to release the funds and stepped in to reassess the buyer. However, completion did go ahead after three days.

In the second case, completion should have been on Monday this week, but has still not happened.

The purchaser, for whom The Partnership is acting, had no idea that she would not be moving on Monday and is paying a removals firm £300 a day for her furniture to sit in a van.

“Fortunately, she has not been hit with a Notice to Complete, which carried with it an automatic £150 cost.

“Once a Notice has been served, the buyer also becomes liable for the other side’s costs, which can rack up at an alarming rate if, for example, the sellers are having to stay in a hotel, have all their meals in restaurants, and make other arrangements.

“A delay of four days could cost as much as £5,000.”

He said of his current client that she had no idea whether the transaction would actually complete, or would be cancelled, and that her stress levels were going through the roof.

Ambrose said: “If a transaction that has exchanged does not complete within two weeks, the contract is then null and void, and both sides can walk away from it.

“But that is only the start of it. The buyer, for example, stands to lose all their deposit, and there might be court proceedings.”

Ambrose added: “Lenders are behaving particularly badly at the moment.

“For example, we lost a client this week because Nationwide would not let us act for them as we are not on Nationwide’s panel. Yet we would have been acting for the seller, not the buyer.

“Santander is quoting two weeks simply to do a credit check on a mortgage applicant before they will even start the valuation process. Yet a credit check probably only takes seconds to run.

“Another client of ours was subjected to a telephone interview lasting two and a half hours with First Direct. Yet this client is already with First Direct, so is a known quantity.

“She said the questions they asked were absurd: for example, she was asked how much her TV licence cost each month. When she said she paid it annually, she was told that it was the monthly cost that was needed.”

Eye asked Halifax to comment on the claim that it is stepping in after exchange and it gave us this statement:

“Our policy is that any applications submitted prior to the 26th April will still be valid for 90 days. If there are any key changes to the application or to the customers’ income or expenditure, the application will need to be reprocessed to be MMR compliant.

“With regards to these particular cases, without knowing the details, I cannot advise you of why they have been reprocessed.”