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Martin sent the urgent warning on The Martin Lewis Money Show where he explained how to cash in on end-of-year tax rewards, which can benefit married couples, first time buyers, people working from home, and those with savings in the bank.
He said: "Right now, on many things, you can go back to the 2016/17 tax year but in three weeks' time, on 6th April, you'll only be able to go back to the 2017/18 tax year. That means, if you don't do something quick, you risk missing out."
You can claim a £250 refund if you got married in the 2016/17 tax year. Marriage tax allowance allows people with an income of £12,500 or less to transfer up to £1,250 of their Personal Allowance to their spouse, if their income is higher.
Martin said this can be backdated for four years. This is your last chance to claim for the 2016/17 tax year.
Working from home has become the new normal for millions of people around the country, and if you are one of them you can claim tax relief up to £125.
Your employer can pay you £6 more per week tax-free to cover expenses. For a basic-rate tax payer, this can be as much as £60, or £125 for a higher-rate tax payer.
"This is in the form of work from home tax relief - and it applies even if you've worked just one day from home in the past 12 months," Martin explained.
If your employer doesn't pay for the uniform you're required to wear at work, you could claim £60 in total due to the uniform tax rebate.
Basic-rate taxpayers can claim 20 per cent which works out to £12 per year while higher-rate ta payers are able to claim £24.
People who are pilots, cabin crew, in the army, RAF, or the Royal Marines should be able to claim more.
Martin added: "If you wash the uniform yourself, you could get up £60 back in the form of a uniform tax break if you backdate it for the full four years."
It's also worth checking to see if you're eligible for a discount or refund on your council tax, especially for those who live alone or if you're a student.
Lots of people end up paying too much money on their council tax if they have been placed in the wrong band. However, you could end up paying more money if you're found to be in too low of a band.
People under 40 can get a £1000 government bonus through a lifetime ISA account. If you use the savings towards your first home, or retirement, you should be eligible.
He added that it has to be open for a year, adding to parents: "If you're a parent and your child is hoping to buy a home one day - possibly in the next few years - open one now, you only need £1 to get started."
The TV licence fee is increasing by £1.50 next year but some over-75s don't have to pay it at all. You are entitled to a free licence free if you claim pension credit. Make sure you tell your elderly relatives and neighbours they could be saving some cash.
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