Mandarins opposed to Britain’s withdrawal from the EU have “taken control” of the Brexit agenda and are “forcing a weak Prime Minister” into a soft Brexit, senior government figures and MPs have warned.

In a dramatic intensification of the war within the Conservative Party, Eurosceptics said that Philip Hammond’s declaration last week that Brexit would be “very modest” appeared to articulate No 10’s “direction of travel” on negotiations.

A Cabinet source warned that Britain faced a “betrayal of Brexit” unless Theresa May reined in Sir Jeremy Heywood, the Cabinet Secretary, and Oliver Robbins, the co-ordinator of negotiations with the EU. David Jones, the former Brexit minister, said: “It’s time No 10 indicated who’s boss.”

In a sign of growing concerns among backbenchers and ministers:

A senior source at the European Research Group of 60 Tory MPs said the group privately shared the concerns about the influence of Mrs May’s officials, adding that they intended to “head them off at the pass”;

Theresa Villiers, the former Northern Ireland secretary, writes for The Telegraph that despite previously making the case for “moderation”, she now fears the Government is heading “towards a dilution of Brexit”;

A leading group of Brexiteers, including the former chancellor Lord Lawson, warn in an open letter that time is running out to strike a deal, and call for Mrs May to offer the EU a “Canada-style” reciprocal trade arrangement or walk away from the talks;

Amid mounting concerns about Mrs May’s leadership and ability to manage the disagreements within her Cabinet, a source at the Conservatives’ backbench 1922 Committee said grandees would warn the Chief Whip in a meeting on Wednesday that she must rein in her ministers.

The interventions come ahead of a meeting on Monday of Mrs May’s Brexit war cabinet, which is split between five ministers seeking maximum “divergence” from the EU’s rules after Brexit and three, led by the Chancellor, resisting a significant departure from the status quo. Each side is lobbying Mrs May and David Lidington, her de facto deputy, for their support.