Agents have been warned that transactions could take longer, with home buyers advised to brace themselves for a longer, more tortuous, and far more personally intrusive mortgage application process.

People at a very early stage of the home-buying process could also be usefully advised to start preparing as soon as possible, by limiting their spending on luxury items and holidays, and clearing any debts.

One broker has even advised that couples could usefully delay starting a family until after a mortgage has been offered and agreed.

The Mortgage Market Review (MMR) is due to start in exactly one month’s time, on April 26, but a number of lenders are either already operating to the new rules or will implement them before the deadline.

Yesterday, trade body the Council of Mortgage Lenders said they would be the largest changes to regulation for a decade.

The CML warned that the “sheer scale” of changes could affect transition to the new rules “in the short term”.

Almost all residential mortgages will have to be sold on an advised basis, rather than execution-only. An advised sale will mean an interview which could take two hours , and perhaps longer, and could involve two separate interview sessions.

The new rules are very prescriptive, with lenders and brokers having to ask more questions to determine the most suitable mortgage and the applicant’s ability to repay should their circumstances change, for example through redundancy or retirement, and/or interest rates go up.

Applicants will be “stress tested” against a rise in interest rates of at least 1% over the next five years.

Home buyers should also be aware they will need to have supporting evidence to hand, such as pay slips, evidence of any overtime or bonus payments, bank statements, and any pension projections.

Crucially, applicants will be asked detailed questions to assess their affordability. These will cover their spending habits, including on pets and meals out, and whether they gamble or have expensive hobbies.

Applicants will also need to provide evidence of, for example, credit card debts, and what they spend on utilities, running a car, travelling to work, plus other living costs such as clothes, childcare, food and recreation.

There has been considerable speculation that the new stress test requirement in particular will lead to more applicants being turned down for a mortgage.

Trade body the Intermediary Mortgage Lenders Association says that nearly six in ten lenders are concerned about the new stress tests, while the City regular, the FCA, is proposing to make the tests tougher.

Broker Aaron Strutt, of Trinity Financial, said anyone planning to take out a mortgage should start planning three to six months in advance and, if they are planning to have a family in the near future, should consider deferring it until they have a mortgage, before loss of income and the cost of childcare can be taken into consideration.

Paul Winter, chief executive of the Ipswich Building Society, said that around one-third of borrowers who have until now been approved for a mortgage could be turned down because of the new affordability testing. He says that the mortgage industry is expecting that from April to July, 67,000 applications will be rejected.

The new rules will affect all mortgage applications, including remortgages and transferring an existing loan into a new property. Only buy-to-let loans will not be affected.