Thousands of Scots face decades of poverty in retirement, either because they have misunderstood how much they need to save or, worse still, they haven't even thought about it.

More than four out of ten questioned by Scottish Widows admitted they hadn't considered how they would survive when they gave up work. Almost as many optimistically said they would look to their children for financial support, while one in seven expected the state to cover their costs.

Many of those who have calculated their requirements have got it badly wrong. According to Aviva, 50 to 65 year olds underestimate the length of their retirement by up to eight years. Women put the average lifespan for a reasonably healthy person at 84 years, while men say it is 80, but they could well live to 89 and 88 respectively.

Clive Bolton, Aviva's managing director of retirement solutions, warned: "Even underestimating life expectancy by a couple of years could have serious consequences for someone in their later years who has outlived their savings, has care needs and has nothing to fall back on."

And the problem is going to get worse. The Office for National Statistics predicts the number of centenarians will rise from 14,000 in 2013 to 111,000 by 2037 and that a third of babies born today will live to be 100.

Meanwhile, just when people need to save more, many have actually reduced what they put away. A survey by HSBC found that despite almost six out of ten UK workers worrying they won't have enough to retire on, the economic downturn has prompted more than four out of ten to cut their pension savings or stop altogether.

Caroline Connellan, the bank's head of UK Wealth, said: "If there's one action we should all consider, it's to start saving as early as possible: even the smallest amounts saved now can make the likelihood of a comfortable retirement all the more real."

No one should expect to live well on a state pension alone. Although the system is being simplified from April next year, with the Additional State Pension abolished for new claimants, the amount people get will still be meagre.

Eligibility for the new single tier pension will be based on National Insurance contributions, with a minimum of ten full years required to get anything at all. Even with a complete NI record - which has been set at 35 years - the maximum individual pension is expected to be around £150 a week, or £7,800 a year.

Anyone currently paying a lower rate of NI because they are "contracted out" of the additional pension, will get slightly less than this under the new scheme.

If you are uncertain of your status, look at your payslip. An "A" on the NI line means you pay into the additional scheme; a "D" or "N" means you are contracted out and will pay slightly more NI from April 2016, when this system ends.

Anyone with fewer than 35 qualifying years of NI payments will also get less pension, so if you have any doubts about your record, check it while there is still time to fill any gaps. To get a statement, go to www.gov.uk/check-national-insurance-record.

Even if you are in a workplace or personal pension scheme, don't assume this will give you enough income. Auto-enrolled employees begin by paying 0.8 per cent of earnings between £5,772 and £41,865 a year, with tax relief adding 0.2 per cent and their employer a further 1 per cent.

This means someone earning £25,000 pays just under £13 a month, or £154 a year, increased by tax relief and employer's contribution to £385.

This will eventually rise to 4 per cent from the employee, 1 per cent in tax relief and 3 per cent from the employer, giving a contribution of 8 per cent. For someone on £25,000, this will make a total investment of £128 a month, or £1,538 a year.

This isn't likely to be worth much more than a couple of thousand a year in retirement for most people, so it is vital to make additional voluntary contributions or pay into a personal plan - as the self-employed must continue to do - to have any hope of living comfortably.

For many, this is made more difficult by other commitments. Just over half of those taking part in the HSBC survey said they simply couldn't afford to save enough and a third said paying off debts was preventing them.

According to Prudential, a fifth of those planning to retire this year still have debts averaging just under £22,000, which will further diminish their standard of living.

To avoid sharing their fate, clear as much debt as you can as quickly as you can. Rethink your other spending to free up as much cash as possible to invest for the future - and keep a regular eye on how it is building up.

If you are in any doubt about what to do for the best, get independent advice. This will become even more important from April 6, when another major pension reform comes into effect.

The legal requirement to buy an annuity with the bulk of a pension pot - consigning people to a low fixed return for life - will be abolished, and savers aged 55 and over will be free to do what they like with their accumulated cash.

Mr Bolton said: "The type of retirement people enjoy will depend on their personal preparation. Retirement won't just happen, it will have to be carefully planned."