By Hester Plumridge 

LONDON-- AstraZeneca PLC's sixth-largest investor, Legal & General Group PLC, is urging the U.K. drug giant's board to reverse its rejection of a $120 billion takeover offer from Pfizer Inc. and engage in talks, two people familiar with the situation said.

The investment arm of the U.K. insurer, which holds a 3.5% stake in AstraZeneca, has written to the company's board asking it to engage in talks with Pfizer, one of the people said. Legal & General declined to comment. AstraZeneca declined to comment on individual shareholder interactions but said it is continuing to engage in general with its investors.

AstraZeneca on Monday rejected a "final" takeover offer from Pfizer worth GBP55 ($92.89) a share. Under U.K. takeover rules, the companies have until May 26 to enter talks or the offer will expire.

Legal & General joins AXA SA, Schroders PLC and Jupiter Fund Management PLC as large AstraZeneca investors that have expressed disappointment at its rejection of the offer. Combined, they hold around 7% of the stock.

Schafer Cullen Capital Management Inc., a New York investment firm that says it holds around 1.5% of AstraZeneca stock in accounts and funds managed on behalf of its clients, said it told AstraZeneca Tuesday that it also believes the board should engage with Pfizer.

"As long-term shareholders we view the Pfizer offer from a price-consideration standpoint as attractive and would encourage AstraZeneca's board to enter formal negotiations to discuss business operating and regulatory details," the firm said in a statement.

Jim Stride, head of U.K. equities at AXA Investment Managers U.K., also urged engagement in a statement Wednesday. "Many shareholders--but not necessarily all--will find this an attractive offer," Mr. Stride said of the GBP55 proposal. "Accordingly we believe that the board...was arguably wrong and acted too hastily to dismiss the latest proposal from Pfizer."

One top-30 investor told The Wall Street Journal Wednesday, "I think that the AstraZeneca board is under reasonable pressure from a sizable chunk of its shareholders."

However, other investors, including Investor AB, Aberdeen Asset Management PLC, Fidelity, St. James Place manager Neil Woodford and Threadneedle Asset Management Ltd.--which have combined shareholdings of around 8.5%--have spoken publicly in support of the AstraZeneca board.

A spokeswoman for Threadneedle said Wednesday the company continued to support the board's stance. "We feel the full implications of the proposed acquisition haven't been sufficiently understood and addressed by Pfizer," she said.

Fidelity Chief Investment Officer Dominic Rossi said he thought Pfizer wasn't a "suitable partner" and that the deal was motivated by tax and finance considerations. "The Astra board has taken a very difficult decision," he said. "They understood in rejecting the offer they would be criticized by some shareholders."

Many other investors, including top-five shareholders BlackRock Inc. and Wellington Capital Management Inc., have declined to comment on their views of the board's actions.

It would be a huge task to get enough shareholders to pressure the board into reversing its position by 5 p.m. BST Monday--just 2 1/2 working days away. Monday is a public holiday in both the U.S. and the U.K.

AstraZeneca shares were up 2.1% in afternoon trading Wednesday, but at GBP44 they remained some way below Pfizer's GBP55 final offer. If the companies don't enter talks by Monday, Pfizer must wait six months if it wants to return with another bid.

Under the U.K. takeover rules, Pfizer is permitted to change the terms of its offer in the remaining days: increasing the cash component from 45%, for example, or adding a contingent-value right--perhaps a structure in which AstraZeneca could get additional shares or cash in the event of an experimental drug being approved. However, it isn't allowed to increase the value of its total offer above GBP55.

Although it is possible that shareholders might pressure the AstraZeneca board into engaging with Pfizer, or that Pfizer might fiddle with components of its offer, "it seems increasingly likely that there will be no further developments for 3-6 months beyond the 26 May," analysts at Barclays wrote in a note to clients Wednesday.

-Jonathan D. Rockoff contributed to this article.

Write to Hester Plumridge at Hester.Plumridge@wsj.com

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