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Billionaire investor and activist Carl Icahn has put technology giant Apple in his sights once again.

Following a semi-successful campaign to encourage Apple to buy back its shares and return more money to shareholders, Mr Icahn is again urging a similar strategy.

In the letter to Apple chief executive Tim Cook, Mr Icahn argues Apple is undervalued.

Apple expanded its "capital return" plan to $130bn (£80bn) this year.

The company said in response to Mr Icahn's letter: "Since 2013, we've been aggressively executing the largest capital return program in corporate history.

"As we've said before, we will review the programme annually and take into account the input from all of our shareholders."

Shares in Apple were up 1.5% in early trading in New York.

, Mr Icahn argued that Apple's stock was truly worth $203 per share - or more than double its current value.

Mr Icahn, who currently owns 53 million shares in Apple or less than 1% of the firm, said he would not sell his shares if Apple did undertake a buyback effort intended to boost its stock price.

In October 2013, Mr Icahn demanded that Apple repurchase $150bn of its shares.

In February, the shareholder advisory firm ISS recommended that investors vote against Mr Icahn's proposals and he eventually withdrew his campaign.

However, in April, Apple's board voted to spend an addition $30bn in share buybacks, as well as approving a seven-for-one stock split, which was the firm's first split in nine years.

Apple did not attribute its decision to Mr Icahn, but some saw it as a minor victory.

Now Mr Icahn is once more on the offensive.

"You have said before that the company likes to be 'opportunistic' when repurchasing shares and we appreciate that," wrote Mr Icahn.

"With this letter, we simply hope to express to you that now is a very opportunistic time to do so."