The world's two largest beer giants have secured a week's extension to agree their £68 billion mega-merger after SABMiller accepted a sweetened offer from rival and Budweiser maker Anheuser-Busch InBev.

Belgium-based AB InBev said in an update it had completed a key phase of its due diligence, but needed more time for discussions to conclude what would be the largest takeover of a British firm in corporate history.

The UK Takeover Panel has extended the time allowed for AB InBev to make a formal offer for SABMiller to 5pm on November 4.

It is the second extension the companies have sought after the original deadline of October 14 was extended to October 28.

The key elements of AB InBev's £44-a-share proposal remain unchanged for a merger which would create a global beer giant worth more than £180 billion, adding SABMiller's beer brands to AB InBev's 200-strong stable, including Corona, Beck's, Leffe and Hoegaarden.

It is thought the two firms would need to shed a raft of assets to overcome regulatory hurdles.

AB InBev is the world's biggest beer business, while SABMiller is the number two.

If AB InBev cannot get the green light from regulators for the deal, or if shareholders do not back the takeover, the brewer would have to pay SABMiller a break fee of three billion US dollars (£2 billion).

The proposed offer is the fifth made by AB InBev in recent weeks, with the group having already tabled bids worth £38, £40, £42.15 and £43.50 a share.

SABMiller had previously spurned its rival's advances, claiming the offers ''substantially undervalued'' the London-based business.

The latest offer represents a 50% mark-up on the closing price of SABMiller's shares before the bid battle started in mid-September.

AB InBev had always said it did not want to go hostile with its takeover plans, but was becoming frustrated by SABMiller's tactics and last week urged shareholders in SABMiller to force its board into serious takeover talks.

SABMiller chairman Jan du Plessis had called his firm the ''crown jewel of the global brewing industry'', adding that previous bids were ''opportunistic and highly conditional proposals''.

As well as being the biggest takeover of a British company, it would also be one of the biggest deals in the world.

But Louise Cooper at financial consultancy CooperCity recently warned there could be a cost to staff if the deal goes ahead, with hefty cost cutting and job losses likely.

She said: ''The more that the bidder offers, the more cost cutting needs to happen to make the deal work financially.

''SAB executives have done a good job for their shareholders - and their own executive compensation schemes - by driving up the bid price. But they have not done such a great job for their employees, more of whom will have to lose their jobs.''

SABMiller has brands such as Miller, Foster's, Coors and Bulmers cider. It employs around 69,000 people in more than 80 countries and has global annual sales of more than 26 billion US dollars (£17 billion).

AB InBev has a 155,000-strong global workforce and makes more than 47.1 billion US dollars (£30.5 billion) in global revenues.

SABMiller attempted to acquire rival Heineken a year ago, but its advances were rebuffed.