The president of the Royal British Legion could be stripped of his role and a crackdown on lobbying of the Ministry of Defence imposed following claims that former senior military personnel boasted of their access to ministers, senior civil servants and top officers.

Philip Hammond, the defence secretary, has launched an immediate review of rules governing access to ministers and serving officers to "shut down" meetings that could be seen as inappropriate.

Labour demanded the MoD disclose whether any of the individuals had "met any MoD officials dealing with any defence equipment contracts".

Though Hammond insisted no contracts had been awarded as a result of outside influence, he said some of the allegations made in the Sunday Times were "deeply damaging to the individuals concerned and their reputations".

In an undercover sting, reporters from the Sunday Times approached several former senior members of the military purporting to be representatives of from a South Korean defence firm seeking to sell an unmanned aerial vehicle, or drone, to the UK military.

Those fooled included Lieutenant General Sir John Kiszely, president of the Royal British Legion; Lord Dannatt, former head of the army; Admiral Trevor Soar, former head of the Royal Navy fleet; Lieutenant General Richard Applegate, a former head of procurement at the Ministry of Defence; Lord Stirrup, former chief of the defence staff, and General Sir Mike Jackson, former head of the army.

During meetings that were secretly filmed, Kiszely said his role at the legion gave him access to important figures in defence, and described the annual Festival of Remembrance as "a tremendous networking opportunity".

At other events he found himself "standing there waiting for the Queen with nothing else to talk about to Philip Hammond than whatever", he is recorded as telling the reporters. He also claimed he would be having Christmas with the armed forces minister, Andrew Robathan. This was denied by the Ministry of Defence. General Applegate is alleged to have said he had successfully lobbied for an Israeli defence firm, even though he was banned from doing so under the rules set out by the advisory committee on business appointments (Acoba). These say former defence personnel cannot lobby for two years after they have retired.

Lord Dannatt said he could speak to the MoD's top civil servant, Jon Thompson, a former school friend. Admiral Soar suggested he could ignore Acoba rules if he was described as a consultant rather than a lobbyist. They all deny wrongdoing.

The Royal British Legion said on Sunday it would undertake an investigation regarding Kiszely's position. Director general Chris Simpkins, said the legion had "a code of conduct that addresses the issue of conflict of interest. The honorary position of national president falls within this code and should not be used for commercial gain or in order to gain access to politicians and senior military figures for purposes unrelated to his position as national president."

Though Simpkins insisted the lists of guests for the Festival of Remembrance was not influenced by the national president, he said the matter would be referred to the legion's governance committee for review. "The findings of the governance committee will be reported to the board of trustees for any necessary action," he added. The MoD inquiry, which will start on Monday, will seek to establish whether Soar and Applegate broke the Acoba rules. Hammond said tougher regulation of meetings between serving and former military personnel might be necessary, which may lead to the extension of the two-year "purdah''.

"There are many, many reasons why it is sensible for the MoD to maintain contact with retired officers. They are often asked [by the media] to comment on things that are going on in the defence area.

" But if they are abusing that access for commercial purposes then we will have to tighten it up or maybe even shut it down. That is something we will now look at. Clearly at least one, possibly more, of the individuals named … were still under the terms of the two-year restriction that applies after they have left the service.

"If we find that people are not abiding by the advice they are given then we will need to look at whether there are steps we can take to shut down their access to ministers and officials." He accused some former officers of "bigging up their capabilities", though he also warned arms firms that use lobbyists.

"The kind of lobbying being talked about here will damage them rather than benefit them … former military officers have no influence over what MoD contracts are awarded."

A senior defence official added: "One wonders what these arms firms think they are getting for their money."

The advisory committee – which has faced demands to be broken up and replaced with a more powerful watchdog with powers to punish rule breaches – said the allegations were "serious" and should be urgently investigated.

"We are drawing them urgently to the attention of ministers, the head of the civil service and the permanent secretary at the Ministry of Defence for them to consider what action they might take."

In a statement, Lord Dannatt said he had "never been asked to lobby and have no intention of lobbying. In my experience lobbying is invariably counter-productive." He said it was "entirely appropriate" for retired senior officers to use their previous experience to advise private companies, but added: "I have no intention, no inclination, and certainly no plans, to engage in lobbying activity that would contravene the rules prohibiting contact between private companies in the final stages of bidding for government contracts, and the officials in charge."

Lord Stirrup said the story had given a "totally false picture" of what he was saying he could do. "I was interested because they were purporting to be developing technologies that could save people's lives. I said I could facilitate conversations with people in the military. I did not offer to lobby anyone on their behalf."

SThe shadow defence secretary, Jim Murphy, said: "These are grave allegations that have to be fully investigated. We need to know every detail, every meeting and every contact between those recorded in this video and those still serving in the MoD.

"The government must have full disclosure about any dealings between these retired distinguished officers and government ministers, civil servants or senior members of the armed forces."