Headteachers have been left angry by a letter from ministers reminding them to provide pupils with nutritious food in school just days after MPs rejected a campaign to extend free meals to children during the holidays.

A letter – from children’s minister Vicky Ford and junior health minister Jo Churchill – was sent to school leaders on Friday morning to encourage them to offer a healthy and nutritious hot meal to their pupils during term-time.

Heads received the letter two days after a Labour motion to offer free school meals to poorer children during holidays – which was prompted by a campaign by Marcus Rashford – was defeated by 322 votes to 261.

School leaders on social media said the letter was “unbelievable”, adding that they were “fuming” and were “speechless”.

Headteacher Andy Byers, who shared the correspondence on Twitter, told the PA news agency that there was anger and “plenty of incredulity” at the timing of the letter from the Department for Education.

It came as a growing number of local authorities – including Liverpool City Council – and pubs, cafes and restaurants announced they would provide meals for local children during the October half-term.

Mr Byers, head of Framwellgate School Durham, tweeted: “Without a hint of irony, Headteachers have received a letter today from the children’s minister reminding us about food standards and our responsibilities to provide students with nutritious food and hot meals. Honestly!”

The letter on school food provision, shared with PA, said: “The vast majority of children are in school and we understand schools may be finding it hard at this time to provide children with their usual hot meal at lunchtime especially given the need to minimise contact between bubbles.

“Furthermore, higher numbers of parents may also be sending their children in with a packed lunch.

“Schools have flexibility within the School Food Standards around which products/foods they serve across the school week. There is no legal requirement that meals should be hot, however we do encourage this and ask all schools to routinely be offering a varied hot meal option to pupils.

“It is much harder to comply with the School Food Standards through the provision of cold food alone. I would urge you all to work closely with your caterers to make every effort to ensure children are receiving a healthy and nutritious hot meal at school.”

Kevin Courtney, joint general secretary of the National Education Union (NEU), said: “Local authorities, schools and businesses in communities across the country are now stepping in where Parliament has not. This is not, however, sustainable long term and the funding needs to come from Government.

“Adding insult to injury the children’s minister has sent out a letter to heads today reminding them of the importance of promoting healthy eating in schools. You couldn’t make it up. Learn the lesson and reverse the decision is the only way Boris Johnson must now proceed.”

A Department for Education spokeswoman said: “All schools have gone to extraordinary lengths to get children back into education this term, and the letter was first and foremost to thank school staff for their efforts in getting kitchens open and once again providing meals to pupils, including those eligible for benefits-related free meals.

“We recognise the pressures in schools during this pandemic and so we wanted to reassure them that we will not be introducing changes to the School Food Standards at this time but that we expect the current robust standards to still be followed and implemented.”