The countdown to the end of the Isa season is on with just hours left in some cases to get your hands on the best deals.

Every adult can save up to £10,200 in a tax-efficient Isa this tax year, which ends on 5 April (see the Full Isa guide).

You can invest the entire sum in a stocks and shares Isa, where most investment gains are tax-free.

You can also save up to £5,100 in a cash Isa which is a normal savings account where the interest is not taxed. Though for every pound in cash, you lose a pound of the investment allowance.

If you're unsure what to do, here are our tops tips.

Don't delay

If you miss the deadline you've lost this year's allowance for good. As an Isa should be the first savings port of call for any taxpayer, it is crucial not to lose it if you have the cash to save.

Once saved or invested, that cash will retain its tax-efficient status for life, unless you withdraw it or the Government changes the rules.

It is estimated savers throw away over £500 million a year by not using their Isa allowance.

Unsure which cash account to choose?

Many of the top cash Isas will soon be pulled so act fast to bag one. Remember, you can only open one account for new money per tax year, though you can open more to transfer funds from another cash Isa.

Here are the current best buy easy access rates in order of when they will be pulled for this tax year.

  • Ends Thursday (online): Santander 3.3% tracker. Its Flexible Isa (issue 3) pays 3.3%, tracks UK base rate for a year but does not accept transfers from old Isas. So if base rate rises, as some pundits predict, so will the return. The online application deadline is tomorrow, or 5 April in branch or by phone.
  • Ends Tuesday: Halifax 3%, top for transfers/consolidation. The Bank's Isa Direct Reward pays 3% (some existing customers get 3.2%). It accepts new money and transfers from old cash Isas. It closes on 5 April at 5pm in branches, 10pm by phone, and 11:59pm online.
  • Ends Tuesday: Barclays 3.25% tracker. The bank's Golden Isa (issue 3) pays 3.25% AER and, like Santander, tracks base rate but does not allow transfers. You must apply in branch before closing time on Tuesday, though Barclays customers with online access can apply online or by phone by 11pm that day.

In the worst case scenario, as long as you open any cash Isa in time, no matter how poor the return, you can benefit.

The key is to beat the deadline as you can always transfer the money to another account later.

For a full list of all best buys, including fixed rate deals where you earn a guaranteed rate for the term, see the Cash Isas Last Chance guide.

Not sure on your investment?

Choosing an investment is a risky business as, if you make the wrong choice, the value of your cash can drop.

If you're in a rush you may not have time to make the correct decision, so advisers suggest you put your money in cash in that event.

Danny Cox, from financial adviser firm Hargreaves Lansdown, says: "If investors are undecided, they can still subscribe to an Isa using a fund supermarket and hold the money in cash in a trading account until they make an investment decision."

Interest from cash in a stocks and shares Isa is taxed but, as Cox points out, having the money secured in an Isa then gives you the opportunity to move it to a good investment.

Many firms, such as Hargreaves Lansdown, will take applications right up to the deadline but check with your chosen provider in case it won't go that late.

Worried you don't have enough to save?

Even if you don't have a large chunk of cash it is still worth choosing an Isa for this tax year.

By beating the deadline it means you won't eat into next year's allowance in case you get a windfall that you can then stash away.

The limit for the 2011/12 tax year, which begins on 6 April, jumps to £10,670, of which £5,340 can go into a cash Isa.

Further reading/Key links

Top deals: Cash Isas, Isa Transfers, Full Isa Guide