A former chief of Nato has launched a powerful attack on hardline Brexiters, accusing them of seeking to use Britain’s armed forces as a bargaining chip to secure a future trade deal with the EU.

George Robertson, the Labour peer who was secretary-general of the western military alliance between 1999 and 2004, said it was vital that the UK continued to be part of a strong European defensive shield, whatever the outcome of the talks on trade.

Robertson, who fears that missteps in dealing with Russia, above all, could have potentially catastrophic results, told the Observer: “There is no doubt, from discussions I have had with some of the Brexiters, that they want to use the British military as part of a bargaining tool. [But] we don’t stop being part of Europe, part of the neighbourhood, or remote from our allies, and certainly not remote from the threat.

“If there is going to be a European capability, then it needs Britain. It can’t be done by the other European countries on their own.”

George Robertson said he thought article 50 letter was ‘clumsily put together’ Photograph: David M. Benett/Getty Images

Russian divisions numbering tens of thousands have amassed just across the EU’s eastern-most border in recent months.

The UK is among a select number of EU member states, alongside the US, deploying hundreds of troops, as well as aircraft and armour, to eastern Europe as part of the biggest build-up of Nato forces in the region since the cold war.

The former Nato chief, who was also defence secretary in Tony Blair’s first administration, spoke out after the prime minister, Theresa May, was accused of threatening the EU with weakened cooperation on security matters in her letter notifying the EU of the UK’s intention to leave.

May wrote that the UK wanted “to agree with the European Union a deep and special partnership that takes in both economic and security cooperation”. The letter then added: “If, however, we leave the European Union without an agreement, the default position is that we would have to trade on World Trade Organisation terms. In security terms, a failure to reach agreement would mean our cooperation in the fight against crime and terrorism would be weakened.”

The link between security and economic cooperation was made 11 times in the six-page letter, and an article penned by the prime minister and published in newspapers in seven countries reasserted the connection. In the Frankfurter Allgemeine Zeitung, she wrote: “This partnership should contain economic as well as security policy cooperation because this is in the interests of the UK, Germany, the EU and the whole world.”

In Brussels, the comments drew a furious response. The European parliament’s Brexit coordinator, Guy Verhofstadt, said it was a “big mistake … to start with launching threats to each other”.

François Fillon, the conservative French presidential election candidate, warned that “Mrs May should not be setting conditions in security matters – we should not get into that sort of blackmail.”

Robertson said he could only assume the letter was “clumsily put together” and “not properly read before it was sent”. He added: “If this is a threat, it is an empty threat.

“The way this government is going about this negotiation is filled with carelessness, which is really regrettable. This is just another indication of the carelessness with which they handle what is going to be a very, very sensitive negotiation. It looks like a clumsy threat.

“The fact is, we will be needed. The problem will be that [after Brexit] we won’t be in when the missions are decided on, we won’t be there when the details are considered, and we won’t be there when the exit strategies are developed.”

The British government rushed to reassure the European council’s president, Donald Tusk, in the wake of the row, insisting that it had not intended to make a threat.

On Friday, on a visit to Nato in Brussels, the foreign secretary, Boris Johnson, said: “It is not some bargaining chip in any negotiations that may be taking place elsewhere in this capital. We make an unconditional commitment to the defence and security of Europe.”

However, the defence secretary, Sir Michael Fallon, insisted that the UK and the EU would both be weaker if they did not agree a Brexit deal on fighting organised crime and terrorism.

“If there is no deal on that, we are all weaker … because that is a joint effort to tackle organised crime and terrorism,” Fallon said.